Implement typed declaration checkpoints and semantic fingerprints

Parent: #8
Blocked by: #31, #32, and #33

Outcome

Create the immutable typed declaration graph and fingerprint contract that connects checking, lowering, diagnostics, and incremental compilation.

Acceptance criteria

• Typed declarations have stable identities, explicit semantic dependencies, and file-level checkpoints that preserve F# source order.
• Exported, implementation, debug, and diagnostic fingerprints are separate versioned values.
• A body-only edit can change implementation without changing exported meaning; public or inline edits change consumer-visible fingerprints; trivia and debug-only edits remain distinguishable.
• Unknown semantic impact is represented explicitly and widens invalidation conservatively.
• Typed nodes retain necessary ranges and resolved symbols but contain no SRM handles, metadata tokens, row offsets, cache serializer objects, or final artifact identities.
• Clean and reused checking produce equivalent typed graphs and diagnostics.
• Fingerprint evidence remains independent of cache layout.

Decision inputs

Issues #17 and #19; ADRs 0013, 0018, 0022, and 0023.

This issue inherits the corpus-neutral acceptance policy in #8.

Start by reading parent issue #8, blockers #31, #32, and #33, then review issues #17 and #19 and ADRs 0013, 0018, 0022, and 0023. Use the acceptance criteria as the completion checklist: equivalent clean and reused checking, separated fingerprints, conservative unknown impact, source-order checkpoints, and typed nodes independent of cache and final artifact details.