bedrock-kv / bedrock / 636fbf24aa77d1cdf493009601561d69b129aba0

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Enforce the segment pool ceiling the recycler always documented (#211)

Closes bedrock-61c.3 (epic bedrock-61c).

`SegmentRecycler.check_in/2`'s docstring has always promised to delete a
returned segment when the pool is full. `max_available` was fetched,
threaded into `State`, and never compared against anything.

Steady state hides it — one checkout per roll and one check-in per trim
keeps the pool oscillating at the cap. A **trim burst** does not:
`trim_durable_segments/1` splits off every segment below a jumped
durability watermark and checks them all in back-to-back with no
intervening checkout, so the pool keeps each at `segment_size` for the
life of the worker. At the 64 MiB production size a ten-segment burst
pins 640 MiB that is never released.

### Changes
- at the cap, unlink the returned segment instead of pooling it
- reject `min_available >= max_available` at construction — with no
slack a checkout drops the pool to `max - 1`, the min-refill allocates a
replacement, and the check-in then unlinks, so every cycle costs an
allocation and recycles nothing. Silent degradation becomes a startup
error. (`recovery_test.exs` used exactly `1/1`; updated to `1/2`.)
- correct two specs that promised `:: :ok` — `check_in/2` can now fail
on the unlink

### Trade
A later burst re-allocates what an earlier one used to leave lying
around. A metadata-only `fallocate` is the right price for not pinning
the disk.

### Verification
2803 tests, 0 failures. Credo and dialyzer clean.

Adversarially audited by a cold agent, which confirmed delete-safety
(every returned file is closed first, and `rm` is path-equivalent to the
old `rename` for every reader) and that the tests are boundary-sensitive
— mutating `>=` to `>` fails two of them. It also surfaced two
pre-existing bugs, filed separately as bedrock-m7j and bedrock-20c.

7 of 7 new or added lines in 1 file covered. (100.0%)

6786 of 8647 relevant lines covered (78.48%)

1030.1 hits per line