Small reads stop reading whole files

[torstei]

First of the changes from the full-read sweep: the sites where a whole file is read to use a few bytes of it. Each is small and independent.

Site	Was	Now
migrate_rings (every writer open)	read the whole .rings to compare an 8-byte magic	reads the magic; the file only when it is v1
serve::grain_header (every ship turn)	read the whole .grain to keep 16 bytes	reads 16 bytes
ship::tape_end, rotate summary	parsed the whole .rings for the last record	format::read_index_last: header plus one record
Session::coverage (every received chunk)	walked every chunk of the destination: quadratic over a replication	the session keeps its runs as chunks arrive
records sink tick	re-parsed .bark ~4×/s to look for a retention or wal change	stat-gated LivePolicy, as append already is

Session::coverage is exact because nothing in a receive session removes chunks (no retention, rotation or reset there), and the list is seeded from the existing chunks when the session opens.

Behaviour change

The sink used to skip retention silently when the manifest stopped parsing. It now keeps the last good policy and warns once, as append does: never silently unbounded.

Measured

Records sink with retention declared, steady state (strace, ~6 s after startup):

	.bark opens	.bark stats
before	4.3 /s	0
after	0.3 /s	1 /s

I did not trace the residual opens. A live timberfs set wal=true on a running sink is still applied within a few seconds.

Tests

read_index_last equals the end of a full parse (v2, a later longer header, empty, a partial trailing record, v1 numbering); push_seq is the same fold as runs_of; grain_header is the first 16 bytes and None for a missing or short grain; migrate_rings leaves a missing, empty, short or non-v1 index alone. Mutating the session to stop updating its runs fails 7 tests.

Not here

The intake's tick has the same per-second .bark parse. Its wal sync iterates every store under one lock, so gating it needs per-store state; that is a separate change. The structural items (resident index per open store, RingsView for readers, head-drop rewrites) are not in this PR.

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