Fix underscore-pileup collisions with scope-aware resolution

[megaboich]

What & why

When un-minifying with a small local model, the renamer produced identifiers with huge runs of leading underscores — up to ~320 in a real run:

________internalState
___________________________________________index   // ~40+ underscores

The bug

The old collision loop deduped final names globally across the whole file and disambiguated by prepending _. A small local model tends to return the same generic word for hundreds of unrelated variables (index was chosen ~1500× in one run, plus node, link, iterator, …). Every repeat collided with the previous one, so the loop kept prepending _ until it found a free slot.

This is over-conservative. Two variables named index in disjoint / sibling scopes are perfectly legal JavaScript and never conflict — only names visible in the same scope chain actually need disambiguation.

The fix

Replace the file-global taken: HashSet<String> with a CollisionResolver that checks a candidate only against names visible in the binding's scope chain:

1. Same scope / ancestors — via Scoping::find_binding (walks up the scope tree): catches same-scope redeclaration and shadowing that would capture an outer binding.
2. Descendants — via an Euler-tour interval over the scope tree plus a live name → enter-indices index (O(log n) range query): prevents renaming an outer symbol to a name a nested scope already binds, which would otherwise capture the outer symbol's references made inside that nested scope.

Genuine conflicts now get a numeric suffix instead of an underscore prefix. If the candidate already ends in a number, that number is incremented rather than a second one appended:

• index → index2 → index3
• foo2 → foo3 (never foo22)

Behavior change

• Collisions render as index2 instead of _index. Non-colliding names are unaffected.
• No public API change: rename_all_identifiers still returns Result<String, RenameError>.
• The architecture is unchanged — renames still go through Scoping::rename_symbol with the final Codegen::with_scoping materialization; no AST mutation of bindings.

Tests

New coverage (unit tests in collision.rs, integration tests in walker.rs):

• identical names in sibling scopes stay bare (no suffix);
• same-scope and shadowing conflicts get exactly one numeric suffix;
• no pileup across many disjoint scopes (regression for ________index);
• descendant capture is prevented (outer a → x where a nested scope binds x becomes x2, preserving the reference);
• suffix increment-vs-append rules (item2→item3→item4, never item22).

All existing tests updated where they asserted the old _-prefix behavior. cargo test, cargo clippy --all-targets, and cargo fmt --check are green.