Support file-backed TZ values - #2247

Summary

Fixes #1406.

Allow TZ to name an absolute TZif file, with an optional leading colon, and accept colon-prefixed zone names. This applies at startup, through --tz, and when the DSL changes ENV["TZ"].

Keep the explicit validation rather than relying only on Go's cached time.Local: invalid names/files still report the existing diagnostic, and timezone changes still take effect within a running process. Files are loaded through time.LoadLocationFromTZData, preserving transition rules, with the same 10 MiB file-size limit as Go's timezone loader. Explicit timezone arguments to individual functions remain name-based.

The troubleshooting template and generated page now describe the supported file forms instead of the previous workaround. Unrelated generated date, platform, and numeric-output changes are not included.

Validation

• 22 focused unit cases: ten fail on the original implementation and twelve controls already pass; all pass with the fix. Coverage includes transition rules, file reloading, spaces/Unicode in paths, malformed/missing files, unchanged state after errors, and the file-size boundary.
• Three added CLI regressions fail against the original binary; the complete 62-case local-time regression group passes with the fix.
• make dev completes: build, unit tests, all 4,796 CLI regression cases, man-page generation, and documentation generation/build.
• Full package race tests pass on Go 1.26.8 and 1.27.1, with 369 passing test/subtest observations on each. go vet passes on Go 1.27.1.
• make lint with the CI-pinned golangci-lint 2.12.2 reports 0 issues.
• 81 additional actual CLI checks exercise inherited TZ, --tz, runtime changes, normal CSV processing, invalid-value controls, symlinks, and winter/summer conversion using a real timezone file. The two changed functions have 100% statement coverage in the Go 1.26.8 test run.