fix: dont delete dev-mode text files during a one-off generate

[Napam]

Problem

Running a one-off templ generate while a templ generate --watch dev server is up breaks every page the running dev server tries to render.

--watch writes each template's literals to a temporary _templ.txt file; a server run with TEMPL_DEV_MODE=true renders from those files. But generate called deleteWatchModeTextFiles() on every run, including non-watch ones which don't own these files. So a one-off generate deletes the .txt files of a separate running watch session, and every render then fails with a missing-file error. The watch session doesn't recover either. It skips rewriting files whose literals are unchanged.

Repro: start a watch-mode dev server (templ generate --watch plus an app run with TEMPL_DEV_MODE=true that renders from the .txt files), then run a plain templ generate (e.g. a CI/check step that verifies generated code is up to date). The one-off generate wipes the server's .txt files and every page then fails to render until the dev session is restarted.

Fix

• cmd.go: only run deleteWatchModeTextFiles() in watch mode a one-off generate must not delete files it doesn't own.
• eventhandler.go: rewrite a _templ.txt file if it's missing from disk, even when literals are unchanged, so a watch session self-heals.

Tests

Two regression tests in main_test.go; a non-watch generate leaves existing .txt files intact, and a deleted .txt file is recreated on the next regeneration.

My use case

I run a dev server while also running static checks quite regularly (especially when using AI agents). Each check runs a plain templ generate, which was wiping the dev server's _templ.txt files and leaving it unable to render any page until I restarted it.

AI disclaimer

I used Claude to help write the tests and suggest the fix.