* refactor: extract float NaN/subnormal helpers for auditability (H-1.4, H-1.5)

Extract inline NaN canonicalization and subnormal checks into reusable
helper functions: F32/F64_CANONICAL_NAN constants, is_f32/f64_subnormal(),
canonicalize_f32/f64(). Pure refactor, no behavioral change.

Makes the float constant folding code auditable against hazards H-1.4
(non-canonical NaN) and H-1.5 (subnormal flushing).

Trace: H-1
Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

* docs+chore: align stale artifacts and docs with code

Doc drift fixes:
- README, usage.md, quick-reference.md, architecture.md: drop stale
  phase-count claims (12-/10-/11-phase) — describe pipeline by passes,
  not by number. Numbers go stale; descriptions don't.
- README: Z3 verification is on by default ("default = ['verification',
  'attestation']" in loom-core/Cargo.toml), not opt-in. Document
  --no-default-features as the disable path.
- README: replace broken Bazel WASM build command with the cargo
  invocation CI actually uses.
- architecture.md: inline heuristics now match code (call_count == 1
  || size < 10, cap size < 50) — were wrongly stated as size < 20 &&
  call_count > 2 / size < 5.
- docs/analysis/z3-status.md: integer memory ops are Z3-verified;
  document known model limitations (top-of-stack-only equivalence,
  Br/BrIf/BrTable break-semantics, contains_unverifiable_instructions
  silently auto-passing).

Stale safety artifacts:
- H-17 MITIGATED: stack validation re-enabled at lib.rs:6782 by 84ce9f7.
- H-18 MITIGATED: Wave 2 added Z3-based ISLE rule verification (REQ-6,
  SG-3); verification feature is default-on.
- H-19 MITIGATED: PR #53 (ac01a19) and PR #80 (c1eb8f3) cover prior
  ISLE skip cases; rewrite_pure now applies to BrIf/BrTable functions.
- H-20 MITIGATED: Wave 2 swapped 5 iteration-order-sensitive HashMaps
  to BTreeMap (REQ-14, #77).
- DD-4 / REQ-10 aligned: drop "12-phase" / "10-phase CLI" framing,
  describe the canonical pass list without phase numbering.

Rivet metadata refresh:
- AGENTS.md regenerated via `rivet init --agents --migrate`. Stale
  duplicate "Project Overview" content removed (was claiming 144
  artifacts / 55 errors / 3 UCAs; reality is 207 / 9 errors / 25 UCAs).
- .rivet/agent-context.md regenerated via `rivet context`.

.gitignore: bazel-* artifacts, .claude/worktrees/ — silences git
status noise.

Trace: H-17, H-18, H-19, H-20, REQ-10, DD-4

---------

Co-authored-by: Claude Opus 4.6 (1M context) <[email protected]>