src,doc,test: gate alpha version tag behind a macro

[lazerg]

NODE_ALPHA_MAJOR_VERSION is always defined, so the #ifdef guard around the alpha tag was true for every release build and v26.8.0 shipped as v26.8.0-alpha.0.0.0. The tag now hangs off a separate NODE_VERSION_IS_ALPHA flag, mirroring how NODE_VERSION_IS_LTS works, so a stable line just leaves the flag at 0 rather than relying on the three alpha macros being deleted. test-release-changelog also checks process.version against the macros now, and that runs on release proposals where the release bit is set.

This on its own won't fix the shipped binaries, v26.x still needs a backport and a new patch release.

Fixes: #65566

[nodejs-github-bot]

Review requested:

• @nodejs/tsc

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.07%. Comparing base (a00cf06) to head (68ebf14).
⚠️ Report is 10 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #65567      +/-   ##
==========================================
- Coverage   90.07%   90.07%   -0.01%     
==========================================
  Files         751      751              
  Lines      254778   254737      -41     
  Branches    48089    48067      -22     
==========================================
- Hits       229494   229456      -38     
- Misses      16464    16475      +11     
+ Partials     8820     8806      -14     

see 29 files with indirect coverage changes

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