refactor(ts): split packages/ts/src clean at one_function_per_file max_lines = 9 #1055

Step under #1053 (census + wave plan) and #1020 (tracker). Lane T, packages/ts.

Main is clean at max_lines = 12 (PR #1051 takes the config there). The next rung is 9. Measured on a clean clone of origin/main at b2c4651 with the config rewritten to 9 and npx -y testing-conventions@0.0.111:

packages/ts/src/resolve-config-extensions.ts:88: `resolveConfigsExtensionSpecs` runs 11 lines, and `loadExtensionEntries` already holds this file
packages/ts/src/resolve-extension.ts:81: `resolveExtensionPath` runs 11 lines, and `resolvePackage` already holds this file
error: 2 function(s) sharing a file with another function over the 9-line threshold

Two violations, two files. In both cases the module's named export is the flagged function and the holder is a private helper, so the fix keeps the export in place and moves the helper out.

Scope

• resolve-extension.ts: move resolvePackage (and platformSuffixes, its only caller's helper, to avoid an import cycle) into resolve-package.ts. resolveExtensionPath and isBareName stay.
• resolve-config-extensions.ts: move loadExtensionEntries (and the Toml type alias it needs) into load-extension-entries.ts. Both resolveConfigExtensionSpecs and resolveConfigsExtensionSpecs stay.
• Colocated tests move with the code; new resolve-package.test.ts and load-extension-entries.test.ts.

Out of scope

No config change. Per #1053 invariant 1, a split PR never touches a max_lines line — the ratchet to 9 ships separately once every lane is clean at 9.

No behaviour change, no public API change (neither symbol is exported from index.ts), so no PARITY.md update and no migration fragment.