fix: lru local cache eviction #71

[asmyshlyaev177]

Limit local cache size with per-script LRU eviction

Fixes #71

Problem

.wireit/<script>/cache/<fingerprint>/ gains a full copy of the script's
output per fingerprint, and nothing removes them. The README told users to
rm -rf .wireit/*/cache by hand.

Cheap when output is dist/, expensive when it isn't. A repo declaring
node_modules as output reached 73 GB at ~540 MB per entry.

Change

LocalCache keeps the N most recently used entries per script — written by
set() or read by get(). Eviction runs after each successful set().

• Default N = 2. WIREIT_CACHE_MAX_ENTRIES overrides: positive integer, or
  infinity for the old behavior. Parsed like the existing WIREIT_PARALLEL.
• Default is small because an entry is a full copy of output, so N multiplies
  a script's disk usage, and large-output scripts are the painful case. 2 covers
  bouncing between the current state and one other (a branch, an undo).

Recency = directory mtime

No index file, no consistency problem. get() stamps the entry it returns, so a
frequently restored entry isn't evicted for being old — that's LRU rather than
FIFO.

atime is unusable: filesystems are commonly mounted noatime or relatime.

Locking

None added. Eviction touches only the calling script's own cache folder, and
StandardScriptExecution#acquireSystemLockIfNeeded already holds that script's
lock across get() and set(). It skips the lock only for an empty output,
where entries are empty directories.

Housekeeping never fails a script

Stamping and eviction swallow errors:

• unstampable directory (read-only mount, foreign owner) must still return a hit
• undeletable entry (EBUSY on Windows) must not fail a script already cached

Degraded behavior is "cache larger than requested", not a failed build.

Entries are ranked with lstat, not stat, so a broken symlink in the folder is
evicted instead of throwing on every future eviction.

Tests

src/test/local-cache.test.ts, unit-testing LocalCache with explicit mtimes so
ordering doesn't depend on timestamp resolution:

case	asserts
under limit	nothing evicted
over limit	trimmed to limit
get() on older entry	it survives, newer one evicted (LRU, not FIFO)
infinity	nothing evicted
post-eviction	survivor still restorable
evicted entry	get() returns undefined
broken symlink in folder	eviction not wedged
stray file in folder	evicted like an entry

Plus an end-to-end CLI test — 5 runs under WIREIT_CACHE_MAX_ENTRIES=2, 2
entries left — and a cli-options parse test. All run under
npm run test:cache-local.

Compatibility

Previously unbounded caches are now trimmed; set
WIREIT_CACHE_MAX_ENTRIES=infinity to opt out. Existing over-limit folders trim
on that script's next cache write.

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