Use shape-proven F64 facts in guarded loop regions

[proggeramlug]

Summary

Numeric object fields proven by a ShapeId now remain native doubles throughout a guarded loop region. Before this change, reads entered the region fast clone but boxed into generic arithmetic for receiver shapes learned at runtime; the same four-read matrix cells cost 44 or 92–94 instructions per iteration. Afterward they cost 19 or 49–50, with Node-equivalent output and unchanged overwrite controls.

Changes

• Carry exact F64 field-representation facts from static births or the region supplier into the region prime ABI and scoped numeric analysis.
• Enter the fast clone only when the receiver shape and requested representation agree; fall back to the generic clone on misses or deprecated/spilled fields.
• Count actual fast iterations with an R fact, and test the emitted native loads/adds and stale-read fallback.
• Add a changelog entry.

Related issue

n/a — step 5 P7 of the one-shape object-model campaign.

Test plan

• Release compiler/runtime/stdlib build from exact code head 3699de102886963c8267d5b572d654232caea26a; final PR head d7f5e014d9618f05e49b79e2f8d4d9ce82adaa99 adds only the changelog.
• Full serial codegen suite: 1,840 passed, 0 failed, 1 ignored; focused region tests and runtime prime refusals passed.
• Pinned Node 26.5.1 parity for tsc 5.8.2, Zod, and numeric matrix fixtures.
• Three numeric matrix fixtures each execute 200,000 fast iterations with rloop_f_rep=200000; emitted fast IR has native load double and fadd double.
• Same-link-mode nine-cell matrix against merged P4 artifacts: six improved, zero regressed, no output divergence; representative read4 44→19 and read4_stmt 92–94→49–50 instructions/iteration; overwrite 11→11.
• Paired 5× tsc/Zod user-instruction medians: +0.002% / +0.021% versus merged P4, within sample noise.
• Full serial runtime suite and final ABI/GC source gates are running.

Checklist

• Workspace version and maintainer release metadata unchanged.
• Changelog entry added.

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[proggeramlug]

Superseded by #11680 (merged as 073b079), which carries the P7 shape-proven F64 region work.