fix(ci): serialise docs deploys so release tags stop racing main

[CSSFrancis]

The failure

The Docs run for tag v0.5.0 failed at the deploy step:

! [remote rejected] gh-pages -> gh-pages
  (cannot lock ref 'refs/heads/gh-pages': is at 38726cd but expected 0f190b2)

Build docs succeeded — only the push lost.

Why the existing guard didn't catch it

# Only one docs deployment should run at a time to avoid race conditions on
# the gh-pages branch.
concurrency:
  group: docs-${{ github.ref }}

A push to main and a push to its v* tag are different refs, so they go
into different concurrency groups and run concurrently. Both push to
gh-pages; the second is rejected. The guard never applied to the case its own
comment describes.

Why it matters more than a red X

prepare_release adds the new version to switcher.json regardless, so a lost
deploy leaves the switcher advertising a directory that doesn't exist. On
gh-pages today both v0.4.2 and v0.5.0 are missing while the switcher
offers them — those entries 404.

The fix

Split the two concerns:

• workflow level keeps a per-ref group (docs-build-${{ github.ref }}) so a
  new push still supersedes an in-flight build of the same ref — PR iterations
  keep cancelling as before.
• the deploy job takes a ref-independent group (docs-deploy) with
  cancel-in-progress: false, so deploys queue. Cancelling would drop a
  version directory just as surely as the race did.

Backfill

Not included here. Re-running the two Docs runs publishes the missing
directories — keep_files: true makes deploys additive, so nothing is
overwritten. I've already re-run the v0.5.0 one.

🤖 Generated with Claude Code

https://claude.ai/code/session_012TTbBeDrPJBTpruuszC3mD

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.32%. Comparing base (1ea6bac) to head (5ee43d2).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #46   +/-   ##
=======================================
  Coverage   90.32%   90.32%           
=======================================
  Files          39       39           
  Lines        4185     4185           
=======================================
  Hits         3780     3780           
  Misses        405      405           

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