refactor: split process_mode_tuned into per-mode render_* helpers (cc 129 -> thin dispatcher)

[cedric013]

Summary

Closes #770.

process_mode_tuned (rust/src/tools/ctx_read/render.rs) was a 557-line match mode block — cognitive complexity 129, and the hottest ctx_read path. This splits it into a thin dispatcher plus one render_* helper per non-trivial mode.

• Extracts the 5 large arms — signatures, map, aggressive, entropy, task — into standalone functions.
• Introduces a Copy RenderCtx<'a> bundle (file_ref / short / ext / file_path / original_tokens / crp_mode / line_count / task) so each helper takes 2-3 params instead of 10 (stays under too-many-arguments-threshold). tuning is threaded only to the arms that use it (entropy, task).
• The match is now a thin dispatcher; the small arms (raw/auto/full/anchored/reference/lines:/density:/unknown) are unchanged.

Behaviour is byte-identical: the arm bodies were moved verbatim and only rustfmt-reindented (hence the large diff). Each mode is now independently testable and per-function complexity is far lower.

Test plan

• cargo test --lib -- tools::ctx_read -> 70 passed, 0 failed (incl. process_mode_output_is_byte_stable_across_calls determinism test and the per-mode tests)
• cargo build --lib
• cargo fmt --check
• cargo clippy --lib --all-features -- -D warnings clean for render.rs (the only remaining error is the unrelated pre-existing rules_overhead.rs:143 lint on main, fixed by fix: silence clippy::map_unwrap_or in rules_overhead (unblocks Clippy CI) #767)

Notes for reviewers

• Risk areas / edge cases: none intended — pure extraction, no logic change. The byte-stable determinism test and the per-mode tests are the safety net.
• Review tip: the diff is dominated by reindentation (12->4 spaces) of the moved bodies; the logic is unchanged line-for-line within each new render_*.
• Backwards compatibility: output is byte-for-byte identical.

[yvgude]

Reviewed the ctx_read process_mode_tuned refactor in detail.

Verified:

• RenderCtx<'a> is a clean Copy bundle that keeps helper signatures under clippy's too-many-arguments threshold
• All 5 extracted render_* helpers (signatures, map, aggressive, entropy, task) are verbatim moves with rustfmt-reindentation only
• The thin dispatcher correctly delegates with one-liners; small arms (raw, auto, full, anchored, reference, lines:, density:) stay inline — sensible scope
• tuning is threaded only to entropy and task (the arms that need it)
• Byte-identical output — process_mode_output_is_byte_stable_across_calls determinism test passes, protecting prompt-cache stability (#498)
• 70/70 ctx_read tests green
• rules_overhead.rs clippy fix is identical to what landed in #765

No behavioral changes, pure structural improvement. CC 129 → thin dispatcher.

Approved.