Make the Sky Clock's stone and rain look less drawn

[keyxmakerx]

Fixes: none
Security implication: none, drawing only.
Consumer-verified: n/a
Foundry compatibility: n/a. This is a canvas drawing change only, rendered with Foundry stubs in Chromium; the live check is in #210.
Mockup: Sky Clock mockup, version 11, on the Chronicle Sign-offs stone weather card (approved).

What this changes

On the stone weather card, Key Maker said the stone had gone "a bit more cartoony again" and that the rain on it also looked cartoony. This PR makes three fixes.

Chips. They used to be flat, dark, rounded spots on a clean outline. Now they are small angular notches broken out of the outline itself, tapering in through the figure's depth with a soft shadow around them. The foot stays whole.

• Edge light, frost and snow are now worked out from the worn outline instead of the clean letter, so they follow the notches.
• Load-bearing code: crumble() (notch plus ext sweep), shapeOf() (each chip's outward direction) and the MASK snapshot that edge() now reads.

Stone face. The face gets two scales of mottling under the speckle, so its tone varies across the surface. The soft grime blotches are now smaller and harder-edged (grainCanvas, weatherCanvas).

Rain.

• Rain no longer leaves round pale beads on the stone. Wet stone now shows a faint sky sheen near its top and thin dark runs, each with a pinpoint of light (stoneMarks).
• Falling rain is finer and fainter, drawn at two depths (drawFront).

Why

These are follow-ups to #209 from the stone weather card review. The look is still a dark weathered stone in Alfa Slab One, as signed off.

Test plan

• CHRONICLE_DIR=../Chronicle node --test tools/test-sky-clock.mjs: 17 pass, 0 fail
• The full tools/test-*.mjs suite gave 1396 pass and 1 fail. The failure is test-shop-room's "vendored files match Chronicle" check, and it fails the same way on main against my local Chronicle checkout. It is not related to this PR.
• tools/check-comment-clutter.sh: OK
• The real SkyClock was rendered with Foundry stubs in Chromium in 12 weathers (clear, rain, snow, thunderstorm, blizzard, hail, fog, sandstorm, ice storm, autumn leaves, heat wave, luminous sky), with no page errors.
• Manual verification in Foundry (Check: the Sky Clock globe in a real Foundry v14 world #210)
• CI passes (all 4 checks)

Tenet self-check

• T-B1 security: n/a
• T-B2 plugin isolation: the change stays within the module
• T-B3 production UI: the motion level is unchanged, and the globe is still below Full motion
• T-B4 docs: no behaviour the docs describe has changed