feat(dev-fkeys): add F6 test runner menu (Phase 2a)

I extended `dev-fkeys.el` with the F6 dispatcher half of the spec. F6 prompts via `completing-read` between two candidates: "All tests" delegates to `projectile-test-project`, and "Current file's tests" detects the buffer's language by extension, derives the runner command, and pipes through `compile' from the projectile root. C-F6 is the fast path straight to "Current file's tests".

Per-language coverage:

- Elisp source files map to `make test-name TEST=^test-<stem>-`. Elisp test files run with `make test-file FILE=<rel-path>` so a per-helper file like `test-foo--bar.el' runs only its own tests.
- Python source files map to `pytest tests/test_<stem>.py'. Python test files run with `pytest <rel-path>'.
- Go runs the package containing the file: `go test ./<rel-dir>'. Source and test files use the same command since Go test scope is per-package. Limit: this runs every `_test.go' in the package, not just the buffer's file. Phase 2b can refine via test-name discovery.
- TypeScript and JavaScript are detected but punted for v1. The runner-command builder returns nil and the orchestrator signals a user-error rather than guessing.

The F6 binding moved from the Phase 1 stopgap (`projectile-test-project') to `cj/f6-test-runner'. C-F6 is newly bound to `cj/f6-current-file-tests'. M-F6 stays unbound, reserved for Phase 2b's "Run a test..." menu entry.

TDD: 68 new tests across 7 files. Production code split into small testable internals (`cj/--f6-language-detect', `cj/--f6-buffer-is-test-file-p', `cj/--f6-source-stem', `cj/--f6-test-runner-cmd-for', `cj/--f6-current-file-tests-impl') plus two thin interactive wrappers. Smoke tests confirm bindings register on load.

I also updated the module commentary with the Phase 2b plan, the capture-then-filter approach for tree-sitter discovery, and a pointer to Emacs bug #79687. The bug is the predicate-syntax mismatch that breaks `:match' / `:equal' / `:pred' queries on Emacs 30.2 with libtree-sitter 0.26. The fix lives on Emacs master (commit b0143530), targets Emacs 31, and has not been backported to the emacs-30 branch as of today. Phase 2b will use queries without predicates and filter results in Elisp, sidestepping the issue. Mike Olson's `treesit-predicate-rewrite.el' applies the same idea to font-lock if you want it before Phase 2b lands.

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