A parser often learns where the next structures are (a node's children,
a structure's body once its prefix gives its length) before it reads
them one at a time. Over openUrl's restartable reader a structure only
reached after a miss costs a pass, and a round trip, of its own.
- clawhdf5-format: `Storage::hint(offset, len)`, "about to be read by
this operation". Default: nothing (every backend that reads when
asked); `&T`, `Box`, `Arc` and the facade's `FileData` forward it
(shifted past a user block, clamped to the file).
- clawhdf5-wasm `LazyStorage` records hinted blocks it lacks. A pass
that misses nothing ignores them (a hint never adds a round trip); a
pass that misses also asks for them, in file order, while the pass
stays within what is left of the operation's `maxFetch` budget (a
hint never makes a call fail). At most 1 MiB (or a block) per hint
and 65536 blocks per pass are recorded, whatever a file makes a
parser hint. `Operation::attempt` follows hints; the plain
`LazyStorage::attempt` does not. `run_blocking` and the browser's
driver use the former. `LazyStats::hinted_blocks` counts them.
No parser hints yet: results, passes and requests are unchanged.
Tests: hinted blocks come with a miss and never alone (cached ones
skipped, a plain attempt ignores them), and stay within the fetch
budget, a hint past the end or longer than the file harmless.
Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>
