v0.193.0 — the reserve guards the write that fills the disk, and its promise is true (R-181)

B2's capture floor (v0.192.0) was consulted in exactly ONE place —
captureAllRecoveryUnits, which writes a few KB. The two legs that write the BULK
into the same backups/primary/<app> tree, the DB dump and the volume dump, ran
FIRST and unguarded. Measured live on demo-hp 2026-08-03 06:40:03: opengist's
volume dump wrote 2.0 GB with no check, free fell to 1.0 GB, and the floor then
refused the cheap write it had already lost the argument to. Its refusal message
claimed "the previous unit is untouched" — measured false: that app's tar had
gone 182,272 B -> 2,147,666,432 B under a stale manifest. Sixth entry in
CLAUDE.md's table of shipped guarantees the code did not provide.

Fix: ONE admission verdict per app per run (internal/backup/admission.go), taken
before that app's FIRST write and covering all three legs — they write under one
per-app root, which is why one verdict can honestly cover them.

- Lazy, at the app's first write, NOT once at run start: app A's dump can put app
  B under the reserve, so a run-start verdict reads a disk that no longer exists.
- Remembered for the run, never re-decided between an app's own legs — that is
  the split this closes. Reset per run.
- Placed ahead of DumpAppVolumesSafe, which stops the stack as its first act, so
  a refused app is never bounced. After the volume-less check, which has no write.
- Exactly one operator alert per refused app per run.
- Leg order unchanged: volume dumps still precede the capture.

The floor is now SIZE-AWARE: it asks whether THIS app's write would cross the
reserve, not only whether the filesystem is already below it — which is how an
app was admitted at 96% and then allowed to write 2 GB. Estimate = the app's
previous .sql + .tar on disk. No history -> headroom-only, deliberately, and the
alert says so.

A container-based du per volume was MEASURED and rejected: 66 timed runs on
demo-hp guest 9201, median ~355 ms/volume (341-404) on volumes holding tens of
KB — container start-up, not the walk. Decisive on top: docker run needs the
writable layer, so it can fail under exactly the pressure the reserve handles.

The message was NOT weakened; the behaviour was moved so the wording became true.
It now also names which term bound. Every claim is checked against a sha256
fingerprint of the tree it describes, never against the log line.

Still refuses and never deletes: nothing here is generational.

11 new tests through the production functions. The DB leg cannot run without
Docker, so its gate is pinned by an AST walk of backup.go asserting admitApp
precedes DumpOne (strings.Contains is insufficient — a commented-out call still
contains the string). 4 red-proofs demonstrated failing then restored.

REUSE.md

@@ -174,7 +174,8 @@
 | `api.GracefulSelfRestart` | controller/internal/api/selfrestart.go | `(logger)` | Controller self-restart | Detached exit; bootstrap unit re-runs the image |
 | `Settings.AddPendingEvent/DrainPendingEvents` | controller/internal/settings/settings.go | offline event queue | Events while hub unreachable | — |
 | `Manager.SetUnitNotify` + `UnitSpace` (R-158/R-167, v0.191.0) | controller/internal/backup/recovery_unit.go | `(func(stack string, err error, *UnitSpace))` | THE per-app Tier-1 recovery-unit capture failure alert — fires PER APP from `captureAllRecoveryUnits`, loop continues | **OPERATOR-TIER** (`recovery_unit_capture_failed`, in the hub's `operatorOnlyEvents`). **NEVER route it to `backup_failed`** — that type is in `DefaultEnabledEvents` and carries Hungarian copy, so it emails the CUSTOMER about a failure they cannot act on (D-c; R-158's own proposal said `backup_failed` and D-c overrides it). `UnitSpace` is **nil when the target filesystem is unreadable** and renders as *"unavailable"*, never as zeros — "0 GB free" and "we could not look" are opposite diagnoses. No controller-side cooldown: the hub owns it |
-| `Manager.unitFloorBlocked` / `floorVerdict` + `FloorUsedPercent`/`FloorFreeGiB` / `ErrCaptureFloor` (R-165 B2, v0.192.0) | controller/internal/backup/recovery_unit.go | `(stackName) (*UnitSpace, bool)` / `(*UnitSpace) (*UnitSpace, bool)` | THE capture floor that replaces the `mp1` bulkhead — refuses ONE app's recovery-unit capture before anything is written when the target filesystem is below the reserve | **Headroom, NEVER unit size** — a per-unit cap is R-163 rebuilt inside one volume. **REFUSES, never deletes:** nothing here is generational (a unit is one fixed path per app, a DB dump one fixed name), so pruning could only destroy a DIFFERENT app's only local copy — **never repurpose `pruneStalePrimaryDirs`**, which removes ORPHANED dirs from an app that moved drives and has no notion of age. Two terms (97% / 1 GiB) in `fillwatch`'s shape, deliberately BEYOND its critical band (95% / 2 GiB) so the customer is always warned first — pinned by `TestFloorSitsBelowTheCriticalWarningBand`. A nil reading neither refuses nor warns (§8.4). Inject `unitSpaceFn` in tests rather than manufacturing occupancy on a real disk |
+| `Manager.admitApp` / `beginAdmissionRun` / `decideAdmission` / `estimatedWriteGiB` (R-181, v0.193.0) | controller/internal/backup/admission.go | `(stackName) bool` / `() func()` | **THE reserve gate. Call it before ANY per-app backup write** — one verdict per app per run, covering the DB dump, the volume dump and the unit capture (all three write under one per-app root) | **Decided LAZILY at the app's first write, never once at run start** — app A's dump can put app B under the reserve, so a run-start verdict reads a disk that no longer exists. **Never re-decided between an app's own legs**: that is exactly the split R-181 closed (bulk written, capture refused). **Reset per run** via the closer `beginAdmissionRun` returns. **Must sit ahead of `DumpAppVolumesSafe`**, which stops the stack as its first act — a refusal decided inside it has already bounced the app. Fires **exactly one** `unitNotify` per refused app per run. Nil admission set (periodic status refresh) → decides fresh, which is still once per app per sweep. Wiring pinned by an **AST walk** in `TestAdmission_IsWiredIntoEveryProductionWriteLeg`, not `strings.Contains` |
+| `Manager.floorVerdict` + `FloorUsedPercent`/`FloorFreeGiB` / `ErrCaptureFloor` / `floorReason` (R-165 B2 v0.192.0, size term R-181 v0.193.0) | controller/internal/backup/recovery_unit.go | `(*UnitSpace, estGiB float64) (*UnitSpace, floorReason)` | The pure two-question predicate behind `admitApp`: is the filesystem already below the reserve (`floorHeadroom`), and would THIS app's write take it below (`floorSize`)? | **Headroom is about the FILESYSTEM, never a per-unit cap** — a size cap is R-163 rebuilt inside one volume; the size term bounds the *delta*, not the unit. **REFUSES, never deletes:** nothing here is generational (a unit is one fixed path per app, a DB dump one fixed name), so pruning could only destroy a DIFFERENT app's only local copy — **never repurpose `pruneStalePrimaryDirs`**, which removes ORPHANED dirs from an app that moved drives and has no notion of age. Two terms (97% / 1 GiB) in `fillwatch`'s shape, deliberately BEYOND its critical band (95% / 2 GiB) so the customer is always warned first — pinned by `TestFloorSitsBelowTheCriticalWarningBand`. **`estGiB == 0` degrades to headroom-only on purpose** — refusing an app with no history makes the FIRST backup the one that can never happen. A nil reading neither refuses nor warns (§8.4). Inject `unitSpaceFn` in tests rather than manufacturing occupancy on a real disk |
 | `fillwatch.Watcher` (`New`/`SetNotify`/`Check`) (R-167, v0.191.0) | controller/internal/fillwatch/fillwatch.go | `(statePath, logger, targetsFn, usageFn)` → `Check() error` | THE customer fill warning — warns BEFORE a filesystem fills, per FILESYSTEM (never per app: one full disk holding ten apps would fire ten times) | Emits the **pre-existing** `disk_warning`/`disk_critical` pair, which was allowlisted + copy'd + default-enabled with **no producer in any repo** until now — do NOT mint a new type beside it. **Two threshold terms, whichever trips first** (85% / 5 GiB; critical 95% / 2 GiB) because a percentage alone lies at both ends of this fleet's size range. **Edge-triggered on ESCALATION ONLY**, state persisted; de-escalation is silent and re-arms. Hysteresis dead zone between clear (75% / 7 GiB) and warn — pinned by `TestThresholdsKeepTheirHysteresisGap`. **A nil usage read is NEVER a warning** (§8.4). The hub has **no `customerMessages` entry** for either type on purpose — an entry would override the dynamic message and discard the drive label + free space |
 
 ### Scheduler / time / UI