docs(labels): text-effects plan + grill transcript

[rulkens]

Summary

• Adds the grill session transcript for the label outline + glow design (10 questions, Q1–Q10, plus a Final design summary block).
• Adds the 12-task TDD implementation plan derived from that transcript.

Docs only — no code changes. Both files are net-new and live under their established homes (docs/grill-sessions/, docs/superpowers/plans/).

Scope of the feature being planned

Per-label opt-in outlineColor / outlineEmFrac / glowColor / glowEmFrac on the Label type. Outside stroke, OVER blend, overlapping bands (glow extends from glyph edge, outline overlays). Em-fraction units, straight-RGBA colour API (migrates existing Label.color in the same change). Atlas rebake with distanceRange = 16. DebugPanel section with category dropdown for tuning.

Test plan

• Skim the plan top-to-bottom — task granularity, no TBDs, file paths look right
• Spot-check 2–3 of the trickier task bodies (Task 5 buffer pack, Task 6 SDF-units math, Task 7 quad expansion) for correctness
• Confirm the migration story for the two Label.color producer sites (youAreHereSubsystem.ts, poiSubsystem.ts → POI_STYLES.*.labelColor) reads cleanly

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