test(core): JSON Canvas spec conformance suite (#16, slice 1)

[LeslieOA]

Summary

First slice of #16 — spec conformance for the core parse/serialize gate. 33 new tests in conformance.test.ts, complementing the sample-fixture coverage already in serialization.test.ts.

Covers the spec checklist:

• All four node types in one document + type-specific required fields
• Full edge matrix — fromSide×toSide×fromEnd×toEnd (64 combos) parsed and round-trip-preserved
• Colour preservation — preset codes 1–6 and hex values verbatim
• Minimal docs — {}, {"nodes":[]}, {"nodes":[],"edges":[]}
• Malformed-input rejection (18 cases) — bad JSON, non-object roots, non-array nodes/edges, missing/empty id, bad type tag, non-numeric coords, missing type-specific fields, bad edge side/end enums
• Round-trip property parse(serialize(parse(x))) ≡ parse(x) + unknown-field preservation (doc/node/edge) + tab indentation

Plus a bundled all-node-types.canvas fixture (text/file/link/group + a fully-specified edge — the sample lacked a file node).

Why pure-TS

No unified/remark/hast imports, so it runs under the existing ts-jest config with zero mocking. Precondition for the Rust port (#6): these fixtures will validate the Rust parser accepts/rejects exactly what the TS one does.

Scope (sliced deliberately)

• This PR — core spec conformance.
• Slice 2 (next) — Canvas Candy cssclasses enrichment: class→prop mapping, frontmatter forms, displayText stripping, zero-overhead path. Also pure-TS (js-yaml only).
• Deferred — callouts parseCallouts conformance: pulls in the unified/remark ESM chain that breaks under ts-jest. Belongs with a one-time jest-ESM solution, not per-test mocking (we learned that lesson on the parse-cache PR). hasCallouts (pure regex) will be covered in slice 2.

Test plan

• 81 tests pass (33 new), typecheck + lint clean
• No device/native needed — pure logic

Part of #16.

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