fix(cli): fall back to prebuilt TUI bundle when npm fails

[chronikion]

What changes:

Make hermes --tui launch from the prebuilt dist/entry.js bundle when npm install fails on certain Linux environments.

Why would it change:

After hermes update, npm install fails with "Exit handler never called!". The crash causes npm to return non-zero. The existing launcher prints "npm install failed" and calls sys.exit(1).

At first iteration, fix tried a different approach — modifying _tui_need_npm_install() to skip npm when the prebuilt bundle existed and .package-lock.json was missing. That failed: hermes update writes .package-lock.json during its own npm install, before npm crashes on teardown. After next hermes update, hermes --tui, .package-lock.json was present, npm was called again, npm failed again, and the fix's filesystem heuristic never engaged.

How does it change?:

A single targeted change in hermes_cli/main.py, inside _make_tui_argv's existing result.returncode != 0 branch — no heuristics, no guessing. Let npm install run normally. When it actually fails:

• dist/entry.js exists → print "npm install failed; falling back to prebuilt bundle" with the npm error preview, set npm_install_failed = True, continue.
• dist/entry.js missing → print "npm install failed" and sys.exit(1) (historical fail-fast, unchanged).

A follow-up guard if should_build and npm_install_failed: should_build = False skips the esbuild rebuild (which would also fail — node_modules/esbuild was never installed).

The new branch only fires when result.returncode != 0 — npm actually failed. Observation-based, not prediction-based:

• Fresh git clone: npm install runs normally, succeeds, esbuild rebuild runs. Unchanged.
• Up-to-date install: _tui_need_npm_install() returns False, no npm install, only esbuild rebuild. Unchanged.
• Termux cold start: termux_need_rebuild check and Termux-specific paths run as before. Unchanged.
• npm crashes (Rocky 9): was sys.exit(1), now falls back to prebuilt bundle and skips esbuild. New behaviour, gated on actual failure.

How to verify:

1. Before this fix: On Rocky Linux 9 (or any environment where npm teardown crashes), hermes --tui fails with "npm install failed" after every hermes update.
2. After this fix: hermes --tui launches from the prebuilt bundle. The user sees "npm install failed; falling back to prebuilt bundle" with the npm error preview, then the TUI starts.

Tests:

• tests/hermes_cli/test_tui_npm_install.py — 28 passed, 0 failed (3 new: fallback fires on npm fail, no fallback without prebuilt, build still runs when npm succeeds)
• tests/hermes_cli/test_tui_resume_flow.py — 45 passed, 0 failed
• All 73 tests across both suites pass.

Platforms tested:

• Rocky Linux 9.7 (kernel 5.14, glibc 2.34), Python 3.11.15

Related:

• PR #37923 — Docker counterpart (merged): sets HERMES_TUI_DIR for prebuilt path. Author explicitly deferred bare-metal "launcher hardening" as "tracked independently." This PR is that deferred work.
• PR #18036 — esbuild bundle foundation (merged): the prebuilt dist/entry.js that makes this fallback possible.
• PR #40543 — npm install workspace scoping (merged): complementary fix that avoids pulling apps/desktop.
• Issue #30271 — WSL Windows npm resolution failure (open): same failure class. The prebuilt-fallback mechanism would also recover this case.
• Issue #34312 — post-update node_modules breakage (open): same pattern of npm producing a broken tree after update.
• Issue #20739 — Docker TUI bugs (open): overlapping failure surface, especially the staleness-check bug (Fix terminal interactivity #4).