✨ feat(plan): Node AST, strict Planned values, GraphBuildError

[roninjin10]

Summary

The authoring half of @smthrs/plan: the pure Node AST that flow bodies will build, ahead of Graph.build (PR4) and the declared Activity/Flow surface (PR2/PR3).

• Node.ts + internal/node.ts: seven closure-free, serializable variants — Succeed, All, Map, AndThen, FlowCall, ActivityCall, Branch. Deferred functions (map transforms, branch predicates, continuations) live in out-of-band WeakMap tables with fnv1a32-source/v1 digests in the AST, so an edited function re-keys exactly its cone.
• Planned.ts: the Planned<T> branded placeholder for step results at plan time. Property access records reference paths (legal — becomes Ref{from, path} key material); any computation — Symbol.toPrimitive, valueOf, toString, toJSON, function application — throws GraphBuildError naming the node and the fix (Node.map to compute, Node.branch to decide, or pass into a payload).
• GraphBuildError.ts: planned_value_computed, invalid_continuation, invalid_all_member, recursion_requires_boundary (reserved for PR4).
• Design per docs/specs/Concepts/Unified Flow Authoring.md + Trampoline Loops.md (decided 2026-08-11). Deviations from the agent-repo prior art are deliberate: Branch/ActivityCall added, Dynamic dropped (agent-layer concept), lenient proxy made strict.

Verification

• npm run check --workspace @smthrs/plan — clean
• npm run lint --workspace @smthrs/plan — clean
• New: Node.test.ts (13), Planned.test.ts (8), GraphBuildError.test.ts (2); full package suite 6 files / 79 tests, 100% branch/function/line/statement coverage enforced by the run
• vitest.config.ts: focused single-file runs now scope coverage to the same-named source module; the package-wide src/** 100% gate is unchanged for full runs

[roninjin10]

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Review round 1 complete (sol + fable, independent). Findings for this PR were fixed and folded into its commit; the full ledger (15 fixes, 6 resolved-as-designed) lives in the stack's review notes. All 14 branches rebased and re-verified: whole-repo check green, all package suites at 100% coverage, rebuilt tip byte-identical to the verified fix tree.