list and info no longer load every grain to count its chunks

[torstei]

Problem

summarize_store (behind list, info and list --json) called grain::load on each store's grain to read chunk_count() and discard the rest: the whole file, plus a copy of every filter into its own Vec. list handles stores one after another, so its peak was about twice the largest grain, not the sum. On a host with a multi-GB grain that is several GB for a command that is documented as read-only and cheap.

Change

grain::coverage(dir, name) returns the grain's size and chunk count from the commit (#189) when it still describes the file, otherwise from one buffered forward walk. It opens nothing for writing and never creates or updates a commit, so list keeps working on a store it can only read.

Measured

Three stores, grains of 68, 17 and 17 MB:

	peak RSS	wall
before	145 MB	0.17 s
after, warm (commit present)	7 MB	0.00 s
after, cold (no commit, read-only walk)	7 MB	0.20 s

Cold, list still reads each grain once, with a bounded buffer. It created no commit files, and it works on a read-only directory.

Tests

coverage agrees with loading the grain, with and without a commit, and does not write one; it ignores a torn tail; it is None for a missing or non-grain file.

Not in this PR

query --has and serve still call grain::load and hold about twice the grain. They need the filters, so streaming them is a separate change.

🤖 Generated with Claude Code