RUE-1477: remove peer semantic CFG assembly

[steveklabnik]

Summary

• delete the peer whole-program semantic/CFG assembly path
• delete CompilerSession::semantic(), its stable semantic/CFG view family, and the park-aware semantic presentation entry
• expose the actual canonical rooted CFG artifact through the unstable in-tree tooling API
• migrate emit, profiling, fuzzing, oracle, and test consumers to the exact rooted per-function records used by codegen
• add structural tests that prevent either the retired orchestration or semantic facade from returning

Each CFG consumer now uses the owning record's exact type, interner, and string domains. No replacement whole-program state or conversion facade is constructed.

Why

The compiler had two ways to assemble semantic and CFG state, and the supported semantic() API preserved a fictional phase boundary even after the duplicate computation was removed. That made architectural measurements ambiguous and left an attractive insertion point for a second pipeline. This change gives every production consumer one canonical computation path before ADR-0071 implementation begins.

Validation

• production compiler and AIR-profile, fuzz, oracle, and oracle-diff consumers build
• compiler unit suite: 844 passed, 6 ignored
• external public-API suite: 6 passed
• external differential oracle: 4 passed
• external payload-schema suite: 3 passed
• formatting and debug-assertion policy pass
• earlier full premerge run passed 150 targets; its sole affected-targets-tool assertion failure reproduced unchanged on clean trunk, and the corresponding CI job passed

Fixes RUE-1477.