fix: pin chapter README Colab URLs to the release tag + dead filenames

[Jammy2211]

Summary

Part of the Colab link-rot repair (#21; phase tracker PyAutoLabs/PyAutoBuild#124). All 71 chapter README Colab links (scripts/ + mirrored notebooks/) were pinned to /blob/main/, which the release-time URL bumper cannot maintain — and which PyAutoHeart's hardened guard now forbids. They now pin to the current release tag 2026.7.6.649 (the bumper maintains them from here). Three dead targets fixed against the actual notebooks: tutorial_4_complex_source → tutorial_5_complex_source (chapter 3), tutorial_3_pixelizations → tutorial_3_inversions and tutorial_11_adapt_regularization.py.ipynb → tutorial_11_adaptive_regularization.ipynb (chapter 4). The three grandfathered Colab entries leave .url_check_allowlist.txt.

Scripts Changed

None — chapter/root README link fixes and .url_check_allowlist.txt purges only; no scripts or notebooks execute differently.

Validation checklist (--auto run — plan was not pre-approved)

• Effective level: safe (header: safe, cap: feature/maintenance → safe at medium difficulty)
• Plan: on the issue (fix: Colab link rot — pin/repoint chapter README URLs, purge allowlists (phase 2) #21), written at launch, unmodified since
• Gate: tests n/a (no test dirs — documentation/tutorial repos) · smoke n/a (README/allowlist-only diff, no script surface) · review CLEAN · Heart YELLOW within launch-acknowledged reason set (5 pre-existing reasons, no new)
• Link-target verification: every Colab URL in the four repos machine-checked — canonical owner, current tag 2026.7.6.649, target notebook exists; Heart's hardened offline guard (url_check.sh, incl. the new phase-1 patterns) exits clean on all four repos
• Human: plan sound in hindsight?
• Human: diff matches plan (no scope creep)?
• Human: merge, amend, or reject — then log the outcome

🤖 Generated with Claude Code