Rename core.Pool to core.Arena

[jakebailey]

Somewhat of a nit but Pool isn't exactly the right term, as it's really an arena or bump allocator.

[DanielRosenwasser]

Some will work, some won't. But always learning.

[Copilot]

Pull request overview

Renames the internal generic bump allocator type from core.Pool to core.Arena to better reflect its allocation semantics (arena/bump allocator rather than reusable pooling), and updates all call sites across the compiler and language service.

Changes:

• Replace core.Pool[T] with core.Arena[T] across the codebase and rename related struct fields (*Pool → *Arena).
• Remove internal/core/pool.go and introduce internal/core/arena.go with the same allocation API (New, NewSlice, NewSlice1, Clone).
• Update LinkStore and other core subsystems (parser/checker/binder/AST factory/printer/sourcemap decoder) to allocate from arenas.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
internal/sourcemap/decoder.go	Switch mapping allocation from Pool to Arena.
internal/printer/printer.go	Rename comment/source-map state allocators from pools to arenas.
internal/parser/reparser.go	Use nodeSliceArena for slice allocations during JSDoc reparsing.
internal/parser/parser.go	Rename parser slice allocators to arenas and update clone usage.
internal/parser/jsdoc.go	Use stringSliceArena / nodeSliceArena for JSDoc parsing allocations.
internal/core/pool.go	Remove old Pool[T] implementation.
internal/core/linkstore.go	Replace embedded Pool with Arena for stored values.
internal/core/arena.go	Add new Arena[T] implementation (renamed from Pool[T]).
internal/checker/checker.go	Replace checker pools (symbols/signatures/index info) with arenas.
internal/binder/binder.go	Replace binder pools (symbols/flow nodes/lists) with arenas.
internal/ast/ast.go	Replace NodeFactory pools with arenas for node allocations.