Expose bounded pocket size lanes without duplicate leaders (#2211)

[pzfreo]

Addresses the rectangular-pocket width/length lane slice of #2211. Stacked on #2219 in GitHub stack #2229; merge atomically with the stack when every required check is green.

An explicit lane moves only the selected approved pocket size into the shared linear whole-ink solve. The combined pocket leader keeps the remaining approved sizes/depth but no longer repeats the moved size. No-lane pocket output remains the existing combined leader. Infeasible lane placement retains pocket_dim_dropped evidence.

Local checks: 26 pocket-completeness and focused tests; all 7 source-shape tests after refactoring and ratcheting the function-length baseline; Ruff; mypy (196 source files); strict MkDocs; architecture guards; 100% changed production-line coverage on the focused run. The broader #2211 remains open.

[pzfreo]

CI review: the macOS canary failed only the source-shape baseline guard. When this pocket-lane slice was restacked above #2228, its follow-up commit accidentally reintroduced the old _render_slot_dimensions length ceiling that #2228 had correctly removed. I amended this slice to keep that ceiling removed; the 7 source-shape tests now pass here and on the rebased #2222 child. The final combined tree is byte-identical to the previously tested tip. Fresh checks are running on the corrected heads.