docs: close L2b malformed-HIR design gate

[sanohiro]

Summary

• close the L2b-a2-am-r design gate with exact checked-HIR record, producer-correction, placement, header, body, and callable-identity contracts
• add exhaustive HIR validation and runtime native ABI ledgers, including canonical codec goldens and the reviewed implementation split
• synchronize the authoritative specification, condensed spec, rationale, settled decisions, roadmap, and handoff status

Verification

• scripts/test-pr.sh
• author ledger consistency pass: 239 ExprKind rows, 11 Stmt variants, 9 TemplatePart variants, A00-A90, 281 unique keyed native symbols, 57/34 canonical tags, 9 codec goldens, and relative links
• replacement normative task_group example checked with target/debug/alignc check
• fresh independent adversarial preflight review: clean

Scope

This is a design-only prerequisite PR. It authorizes no implementation by itself. The reviewed am-e through am-c sequence becomes project truth only after this PR passes the repository gate and merges.