Compat: approximate diagonal contact trace by padded bounding box (#157)

[loom-fleet-dispatch]

Closes #157

Under --fasthenry-compat, a diagonal contact trace is accepted and refined as its bounding box padded by 3w/2 (a decay_rect/contact rect, no outward limit) with cells (w/2)·s^min(|tan θ|,|cot θ|) (cell capped at half the box's narrower width). A line-numbered warning names the staircase approximation. Native mode keeps the rejection (message reworded). Docs table row, decision section, module docs and CHANGELOG updated.

Tests: 30°/60° mirror images and 45° equal the explicit contact rect; warning text/line; native rejection unchanged. fmt, clippy -D warnings, fasterhenry-cli tests pass.

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[loom-fleet-dispatch]

Judge pass: still carries a fresh loom:reviewing claim (claimed 2026-10-06T00:47:44Z, idle 11m) — standing down without reclaiming. Not stomping.

Stand-down passes against this claim: 4 of 3 before the bounded fallback force-reclaims it. This comment is edited in place on each pass rather than reposted (#5123, #6514).

[loom-fleet-dispatch]

Judge review (explicit request): approved.

Acceptance criteria for #157:

• Compat 30/45/60 deg parse; compat_diagonal_contact_trace_is_its_padded_bounding_box asserts the mesh equals the explicit padded-bbox contact rect with cells (w/2)*s^min(|tan|,|cot|). Met.
• Symmetry: the test covers both [4,a] and [a,4] spans (30/60 mirror) with the same cell, since ratio = min/max of spans. Met.
• Warning names the approximation (on the clause's own line); native rejection unchanged, message reworded, test asserts it. Met.
• Compat-table row, decision section, module docs and CHANGELOG updated. Met.

Checks: inp tests 121 pass locally (private target dir); all CI checks pass (incl. clean-room), merge state CLEAN. Curator guidance followed; the half-narrower-width cell cap is a reasonable documented safeguard.

Non-blocking nits: the old decision section's body still opens with native-only text (fine, now labelled), and there is a stray double blank line after the new compat paragraph in docs/fasthenry-compat.md.

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[loom-fleet-dispatch]

Landing order recorded — this PR overlaps other open work

Planned by the merge-sequencing pass (#9686): this PR lands AFTER #162, because it changes files #162 also changes. Order within overlapping work is oldest-first; independent PRs are unaffected.

While the loom:sequenced label is present, merge-pr.sh refuses to merge this PR (the #9378 gate). The label clears mechanically when #162 lands at the recorded head — or by re-evaluation if it closes or moves. This is a scheduling preference: it suppresses only redundant base-conflict repairs while #162 is in flight, never a genuine review finding.

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Automated by loom-daemon claim reconciliation (#9686, plan seq-2c17c32b)