refactor(core): reuse formatter file extension

[kitlangton]

Why

Each formatting request recalculates the same filename extension for every registered formatter. The extension depends only on the requested path.

What Changes

Compute path.extname(filepath) once before selecting matching formatters. Matching remains case-sensitive and uses the same Node path semantics, registry order, and first-success/fallback behavior.

Scope

The matching expression and focused matching tests only. Command resolution, process execution, timeout policy, and error handling are unchanged.

Verification

cd packages/core
bun run test test/formatter.test.ts
bun typecheck
cd ../..
bunx prettier --check packages/core/src/formatter.ts packages/core/test/formatter.test.ts
git diff --check HEAD^ HEAD

17 tests passed with 24 assertions. New real-harness cases cover matching/nonmatching extensions, case sensitivity, dotfiles, extensionless files, and multi-dot names. Core typechecking, formatting, and whitespace checks passed.