[Compiler Spine] Track remaining phase-hardening follow-ups

[cssbruno]

Source: follow-up audit for #58.

This issue tracks compiler-spine items intentionally deferred after the IR migration, invariant-gate cleanup, release-plan closeout, syntax contributor docs, and generated-Go policy update in PR #236.

Remaining follow-ups:

• Add exact source spans where diagnostics, reports, and LSP metadata still fall back to broader spans.
• Add component graph and asset graph commands, then pin their output with deterministic golden tests.
• Decide whether invalid-IR test helpers should panic in tests, or keep returning explicit internal compiler errors at public generator boundaries.

Acceptance criteria:

• Remaining exact-span work is split by user-facing surface so diagnostics, reports, and LSP behavior can be reviewed independently.
• Component/asset graph contracts are documented before being advertised as stable.
• Any intentionally deferred invalid-IR test-helper policy is documented in the relevant compiler or engineering doc.

Related: #58 and #236.