feat(compile): module-graph tree-shaking / DCE for compilePackages (#2309)

[proggeramlug]

What

Implements the module-graph dead-code-elimination phase from #2309 — the bundler-equivalent step Perry was missing. Both stages, behind a default-OFF flag (PERRY_TREE_SHAKE=1 or perry.experiments.treeShake: true). With the flag off, behaviour is byte-identical to before.

This is general compilePackages infrastructure (helps Hono/Drizzle/Effect/any npm tree), surfaced by the #348 ink recheck: Perry compiles every import-reachable module and hard-errors on new Function / unimplemented APIs in dead transitive code that never runs.

Stage 1 — binding-level reachability + sideEffects prune

• Deferred refusals (perry-hir/src/deferral.rs): while lowering a node_modules module under tree-shaking, new Function (eval_classifier) and the 5 #463 unimplemented gates record the refusal and fall through instead of hard-erroring. Re-raised after pruning only for modules that survive — dead code's refusals are dropped, live code's stay fatal and span-precise.
• Reachability fixpoint (compile/reachability.rs): monotone Rooted/Bindings lattice. User (non-node_modules) modules seed Rooted, so user code is never pruned — we only ever drop dead dependencies. Honors package.json "sideEffects": false: a pure re-export barrel reached only via named bindings follows just the edges feeding the needed exports and drops bare side-effect imports. Every ambiguous shape (namespace import, export *, dynamic import(), unresolvable specifier) conservatively roots the whole target. Surviving barrels have dangling edges to pruned modules rewritten away.

Stage 2 — process.env define-folding + dead-branch elimination

• compile/env_fold.rs: a build-time constant folder over if conditions, run before dynamic-import() edges are registered, so an env-gated dead import() never enters the graph. Resolves process.env.<NAME> from explicit perry.define (esbuild-style, host package.json only) plus an implicit NODE_ENV → "production" default for node_modules code. Folds Compare/Logical/Unary over literals; strict eq/ne total, loose eq/ne + ordering only within matching types (never replicates full JS coercion); anything not provably constant is left as a runtime read.
• expr_member.rs: process.env["NAME"] with a string-literal key now lowers to EnvGet (matching the dot form), so the fold sees ink's bracket access. Runtime-identical to the prior EnvGetDynamic(String) path.

Validation

• Real import { throttle } from "es-toolkit/compat" (the headline CASE A): tree-shaking prunes 430 unreachable modules including template.mjs (the new Function leaf) and the program compiles + runs. Flag-off, the same import hits the new Function wall.
• Env-gated dead import() (CASE B): a new Function module behind if (process.env['DEV'] === 'true') fails without a define (branch kept → leaf reached) and compiles+runs with define: { "process.env.DEV": "false" } — both dot and bracket access.
• Flag-off path verified byte-identical on plain programs.
• Unit tests: 4 reachability fixpoint (barrel-drop / namespace-keeps-all / export*-keeps-source / non-barrel-escalate) + 3 env-fold + deferral. Full perry-hir suite (116 tests) green; cargo fmt clean.

Scope / non-goals

• Makes ink (and other npm trees) compile, not render — yoga-layout (WASM) is genuinely reached and DCE can't strip it. Compile ink (React-based TUI framework) end-to-end via perry.compilePackages #348 stays a smoke test; perry/tui remains the product TUI path.
• Module granularity everywhere except pure sideEffects:false re-export barrels (the only sub-module pruning); ternary import() gating and closures nested inside expressions are left for follow-up.
• Fixes the codegen bug Codegen: module-level let mutated via ++/-- inside a nested fn in a cjs-wrapped module → 'Undefined variable in update expression' #2310 in practice by never reaching it (its ws module is in the env-gated dead branch).

Closes #2309