fix(shim): handle Windows .exe case + seed shim-map at install

[CalvinAllen]

Summary

Two related shim-resolution bugs surfaced by running Python-driven tools (mmdc invoked via subprocess by shutil.which-resolved paths) against a globally npm install -g-ed mermaid-cli:

1. Case-sensitive .exe stripping — getShimName used strings.TrimSuffix(shimName, ".exe") on os.Args[0]. Windows command resolution via PATHEXT (which defaults to uppercase .EXE on many systems) causes Python's shutil.which — and any other tool that preserves the case returned by PATHEXT-based path lookup — to produce paths like mmdc.EXE. The uppercase extension survived TrimSuffix, so the shim name passed downstream was "mmdc.EXE". That key was absent from the shim-map cache (lowercased at build time), the provider-registry prefix match (HasPrefix("mmdc.EXE", "node"/"npm"/"npx") all false), and the provider lookup (no registered mmdc.EXE provider) — producing the misleading runtime provider not found: runtime provider 'mmdc.EXE' not found error for a command that was otherwise correctly installed and shimmed.

2. Install-time shim-map gap — runtime providers' createShims() wrote the shim binaries for the runtime's core commands (node/npm/npx, python/pip/..., ruby/gem/...) but did not update cache/shim-map.json. SaveShimMap was only ever called from the full Rehash path. In practice this was papered over by mapShimToRuntime's provider-fallback — npm resolved via Node.Shims() membership even when the cache didn't know it — but the fallback only matches core shim names and breaks down the moment a globally installed npm package (e.g., mmdc) gets invoked before any subsequent reshim has run.

Changes

Fix 1 — fix(shim): strip .exe suffix case-insensitively when resolving shim name (cdb883c)

• Refactor getShimName → shimNameFromPath (pure function, testable).
• Use filepath.Ext + strings.EqualFold(ext, ".exe") so .exe / .EXE / .Exe all strip.
• Non-.exe extensions (.bat, .cmd) are preserved unchanged.
• New src/cmd/shim/main_test.go with 7-case table-driven test (unix bare, lowercase/uppercase/mixed-case .exe, forward-slash paths, bare names, non-.exe preservation).

Fix 2 — fix(shim): seed shim-map cache during runtime install (7e2ec38)

• shim.MergeShimMap(entries ShimMap) — merges entries into the on-disk map, creating it when absent; resets the in-memory cache so subsequent LoadShimMap reads the merged state. Preferred over Rehash() at install time because it doesn't re-scan every runtime's bin directory.
• Manager.CreateShimsForRuntime(runtimeName, shimNames) — creates shim files and registers them in the cache in one call, keeping the two representations in sync by construction.
• node, python, and ruby provider createShims() all switched from bare CreateShims to CreateShimsForRuntime.
• Bare CreateShims retained for tests and any future caller that only needs file creation.
• 4 new TestMergeShimMap_* tests covering fresh cache creation, disjoint merge, overwrite semantics, and in-memory cache invalidation.

Why two fixes, not one

Each is necessary but not sufficient. Fix 1 alone still leaves users broken on a cold install (cache is empty until the first successful reshim, and Python-invoked shims can't rely on the provider-prefix fallback when the caller is a non-core binary like mmdc). Fix 2 alone still leaves Python-driven invocations broken even after the cache is correctly populated, because case-sensitive extension stripping yields a cache-miss key. Both bugs have to be fixed to get Python-driven Windows tooling working reliably against dtvem shims.

Test plan

• go test ./src/internal/shim/... ./src/cmd/shim/... — all tests pass (11 new cases: 7 TestShimNameFromPath + 4 TestMergeShimMap_*)
• ./rnr check (format, lint, full test suite) — exit 0
• Manual Windows repro: on a clean dtvem install, dtvem install node <ver>, inspect ~/.dtvem/cache/shim-map.json — core node/npm/npx entries should appear immediately (previously would only appear after first reshim)
• Manual Windows repro: npm install -g @mermaid-js/mermaid-cli && dtvem reshim && python -c "import subprocess; subprocess.run(['mmdc', '--version'])" — should succeed (previously failed with runtime provider 'mmdc.EXE' not found because Python's shutil.which returns uppercase .EXE)

Scope notes

• No behavior change on Unix — filepath.Ext on unix-style shim paths returns "", EqualFold("", ".exe") is false, strip is a no-op.
• No API break — CreateShims signature unchanged; CreateShimsForRuntime is additive.
• Extension stripping only — the base shim name is not lowercased. If other case sensitivities surface (e.g., users manually typing MMDC), that's a separate consideration and likely belongs in mapShimToRuntime rather than getShimName.