test: mirror flat unit-test trees into src feature subdirs

[mtskf]

Summary

Mirror the flat unit-test trees onto their src/ feature subdirectories so src↔test correspondence no longer costs a search. Pure move — git mv plus relative-import rewrites inside the moved tests; no test logic, src/, or config changes.

Changes

• test/webview/ (113 files): 80 tests moved into decorations/, fenced-code/, fold/, frontmatter/, image/, inline/, lint/, list/, outline/, paste/, table/, task-checkbox/; 33 cross-cutting / top-level-cm tests stay at the tree root. helpers/ untouched.
• test/extension/ (50 files): 40 tests moved into commands/, config/, conflict/, handoff/, image/, links/, lint/, session/, surface/; 10 top-level tests (extension.activate, package-*, types-equality, webview-*, …) stay at the root. e2e/ untouched.
• Every moved file descends exactly one level, so each relative specifier was rewritten uniformly (../../src → ../../../src, ./helpers → ../helpers, ./vscode-stub → ../vscode-stub), including multiline import() / from and vi.mock / new URL forms.
• No config edits needed: the vitest include glob and the test/webview tsconfig include are recursive; tsconfig.unit.json names only types-equality.test.ts, which stays at the root.

Related

• TODO: 🧹 Mirror the flat test trees into feature subdirectories — test/webview/ + test/extension/

Test Plan

• pnpm compile clean (all 4 tsconfigs)
• pnpm test:unit — 187 files / 3327 tests, identical to the pre-move baseline
• pnpm test:browser — 9 files / 26 tests pass
• pnpm lint clean on the moved dirs
• Resolver audit: every relative specifier in all 121 moved files resolves on disk (catches type-only imports that compile/vitest would strip silently)