bench(catalog-detector): who owns "the listing is owed again"

12a fixed the skills listing by having each writer announce itself, which is
correct and does not scale: the same probe showed a skill file written past
those writers — an editor, a script, the model's own write tool — is in the
canonical registry and still owes nothing. A contract that reads "every writer
remembers to announce" is one forgotten call from the defect it just fixed.

So this measures the three candidates rather than picking one. Rebuild the
projection and compare it; fingerprint path, size and mtime; or trust that every
write announced itself. Owed is measured, not declared: the block is rendered
before and after each transition, and owed means those differ. Under that
definition rebuilding the projection is not an estimate of the truth, it is the
truth, and what the arms establish is where the two cheap detectors diverge
from it.

Both diverge, in opposite directions. Metadata misses a change whose length and
mtime are preserved, which is not exotic — a branch switch restores content and
stamps it with the mtime the file had — and it fires on a touch and on a body
edit, neither of which changes a line the model sees. Observed-write is blind to
every out-of-band write by construction; it can be an optimization, never the
owner of correctness.

The cost argument for the cheap ones then mostly evaporates. Rebuilding the
projection costs 166µs on a workspace with no skills of its own, 668µs at ten,
2.4ms at fifty and 6.4ms at two hundred; metadata saves 25% of that at two
hundred and 60% at ten, because the walk is the cost and reading the files is
not. A turn costs seconds. Paying the truth once per turn is noise, and it was
avoided on an assumption nobody had measured.

Two measurement traps are worth naming. Timing a detector with its own
accounting inside it measures the accounting: attaching the file/byte counter to
the projection detector reported it 2.5x slower than it is. And pointing the
project root and the home root at one directory walks it twice, which doubled
every number until they were separated.

.gitignore

@@ -20,6 +20,7 @@
 /covergate
 /repolint
 /write_heartbeat
+/catalog-detector
 
 # Website (Astro) build output
 /site/dist/