[rust-compiler] Tolerate unknown statement kinds at the AST boundary

[poteto]

Summary

Babel can emit statement kinds the typed AST does not model; #36704 pins three TS module-interop forms. Deserialization previously failed the whole file on the first such node, while the TS reference compiles the file and leaves the statement alone.

• Statement gains a final #[serde(untagged)] Unknown(UnknownStatement) variant carrying the complete raw Babel node. Deserialization is hand-written and dispatches modeled type tags through a KnownStatement helper enum, so a malformed modeled node still errors with its precise field-level message instead of degrading to Unknown; only genuinely unmodeled tags take the catch-all. A known_statements! macro is the single source for the dispatch enum, its From mapping, and the tag list, so the three cannot drift from each other.
• Top-level unknown statements are preserved verbatim through re-serialization. Function-body occurrences record the standard UnsupportedSyntax bailout with an UnsupportedNode instruction carrying the raw node. The TS reference reaches its equivalent default case only via assertExhaustive, which Babel's closed types make unreachable; in Rust unmodeled syntax is reachable by construction, so it degrades per the fault-tolerance model instead of crashing.
• Program-level analyses handle Unknown explicitly: the gating reference-before-declaration scan walks the raw node for identifier references (an export = X does reference X), and the prefilter and return-analysis arms are deliberately inert.
• The SWC/OXC reverse converters get loud tripwire arms (a deliberate throw in generated code) instead of silently dropping unknown nodes. The SWC forward path is fixed in the next PR of this stack.
• rust-port-0001-babel-ast.md's no-catch-all policy is amended to document Statement as the single deliberate exception.

Perf note: deserialization now materializes a serde_json::Value per statement before typed parsing. The marginal cost is a move-based tree rebuild at a one-time boundary; the previous derive also buffered every node through serde's internal Content to read the tag, so the delta is allocation shape, not asymptotics.

Test plan

• cargo test --workspace green; unit tests cover program-level and nested-in-function round trips with reserialize equality, known-tag non-shadowing, precise malformed-node errors, missing/non-string/non-object type, scoped raw mutation, and a lowering integration test pinning the function-body bailout shape
• All three fixtures pass scoped Babel e2e including events parity (import lib = require("shared-runtime") is preserved verbatim next to real memoization)
• Full Babel e2e 1791/1798; the only failures are the 7 documented pre-existing ones
• test-babel-ast.sh (round-trip and scope-resolution tests) fully green on this stack

Stack

PR	Slice	Base
#36517 → #36518 → #36522	SWC parity stack (open)	pr-36173
#36704	TDD fixtures (red)	#36522's branch
#36705	Unknown-statement tolerance (Babel green)	#36704's branch
#36706	SWC preservation + renames (SWC green)	#36705's branch

Merge order: bottom up.

[poteto]

Closing: ported directly to the umbrella PR branch (rust-research, #36173) along with the rest of this stack.