feat(ctx_read): serve explicit re-reads as deltas when delta_explicit is enabled

[parker-brown-family]

Hi, and thanks for lean-ctx 👋

Daily user here. This adds an opt-in I wanted for myself: delta-aware explicit
re-reads in ctx_read.

For context — a lot of my sessions are tight iterative loops over a handful of
files: read, edit, re-read to confirm, edit again. When I (or the agent)
explicitly re-read a file already seen this session, getting the whole thing
back is mostly tokens I'm already holding — the change is the only new
information.

The pain

lean-ctx already collapses auto re-reads to deltas. But an explicit full
or lines:N-M re-read of a session-cached file re-emits content the model still
holds, even when only a few lines moved.

What this adds

delta_explicit (config field + LCTX_DELTA_EXPLICIT env, default off): when
enabled, an explicit re-read of a session-cached file returns mode=diff if it
changed on disk, or collapses to the existing full-mode stub if unchanged. First
reads are untouched; fresh=true always bypasses.

Tradeoff / Tension

Opt-in by design — some workflows want an explicit full to always mean full.
Default off keeps it purely additive: those who want the token savings on explicit
re-reads turn it on. Workflow-specific, like a lot of token-economy choices.

Notes

• Staleness uses the verified mtime+md5 check (is_cache_entry_stale_verified),
  so a same-second write can't produce a misleading empty diff.
• Output stays byte-stable / deterministic (no timestamps in the note).
• The diff base is always the full cached source, never a compressed view.

Tests

Config round-trip + env override, and ctx_read behavior: unchanged → stub,
changed + enabled → diff, disabled → full (today's behavior), fresh=true
bypasses, first read untouched.

CI / back-compat

cargo fmt and clippy -D warnings clean on the changed files; ctx_read and
config lib tests green. One additive config field; default off.

[github-actions]

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[parker-brown-family]

recheck

[yvgude]

Reviewed line-by-line: the core is a pure resolve_explicit_delta_mode(cache, path, mode, …) with no wall-clock/counter/randomness, opt-in (default off = zero behavior change), and #498-determinism-aware — it uses the verified staleness check so a same-second write can't be misread as unchanged, and ships a byte-stability test plus a guard that the diff base is the full cached source (not a compressed view). Thorough test matrix (changed→diff, unchanged-lines→stub, fresh bypass, first-read, off-preserves). CI green. LGTM.