feat(scatter): add bubble variant for three-value comparisons (#141)

* feat(scatter): add bubble variant for three-value comparisons

Bubble chart as a Scatter variant per the issue #62 triage: x and y on
shared zero-based linear scales, bubble AREA proportional to the third
value (r = K*sqrt(size)), one accent bubble, ink-opacity ramp as ink-mass
compensation, largest-first paint order.

scripts/verify-bubble.py derives both axis scales and the area constant
from the set itself (Theil-Sen, leave-one-out) and holds every bound
label and tick against the mark it describes; test-verify-bubble.py
exercises both polarities. Bindings are bubble-specific (data-size,
data-name, data-tick) rather than data-series, so verify-slopegraph.py
never claims a bubble file and neither checker needs scope carve-outs.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* fix(bubble): report a bubble whose peers cannot describe a scale

A bubble holding the only distinct value on an axis was skipped by the
leave-one-out residual test — its own fit is degenerate, and the full-set
fit passes through wherever it was drawn, so a wrong coordinate shipped
clean. Report it as unverifiable instead of skipping it.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* chore: bump to 2.6.3 and register bubble gates

Ridgeline (#143) landed on 2.6.2, colliding with this branch's bump, so
re-bump past it. Also add the bubble gate commands to the maintainer
policy's local_commands, matching what ridgeline registered.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

* chore: bump to 2.6.5

The mermaid fix (#131) landed on 2.6.4, colliding with this branch's
bump again. Re-bump past it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

---------

Co-authored-by: 0xDarkMatter <0xDarkMatter@users.noreply.github.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

.maintainer-policy.json

@@ -53,6 +53,8 @@
       "python3 scripts/test-verify-slopegraph.py",
       "python3 scripts/verify-ridgeline.py --all",
       "python3 scripts/test-verify-ridgeline.py",
+      "python3 scripts/verify-bubble.py --all",
+      "python3 scripts/test-verify-bubble.py",
       "python3 scripts/build-icons.py && git diff --ignore-space-at-eol --exit-code -- skills/diagram-design/assets/icons.html skills/diagram-design/references/primitive-icons.md"
     ],
     "required_check_names": [],