Add styling hooks: custom properties and ::part() on squares

[laat]

Closes #21.

Both shapes proposed in the issue, since they cost nothing together and neither touches the JavaScript API.

Custom properties

The shadow stylesheet now reads these, with the previous hard-coded values as defaults, so nothing changes for existing pages:

Property	Default	Applies to
--chess-board-light	#FFCE9E	Light squares
--chess-board-dark	#D18B47	Dark squares
--chess-board-border	1px solid black	Border around the 8×8 board
--chess-board-frame-font-size	40%	File and rank labels

The frame label size is included because the issue lists it among the values a page could not reach.

Parts

• Every cell: part="square <name> <shade>", e.g. square e4 light, so chess-board::part(e2), chess-board::part(e4) { box-shadow: inset 0 0 0 0.15em #f0d13a } marks a last move.
• The 8×8 table: part="board".
• The file and rank labels (and the empty corner cells): part="frame".

The highlight attribute alternative is left out: parts cover it with no JS, and an attribute can be layered on top later if it turns out to be wanted for discoverability.

Demo

Two examples added to index.html, in the page's existing style: a green board with no border through the custom properties, and a board with the last move (e2, e4) marked through ::part(). The source block under an example now includes the CSS from a <style> inside the example, since these two need it.

Verification

• Three new tests check the part names on the table, on a8, d4, e4 and h1, on all 64 cells and all frame cells, and that the stylesheet reads each custom property with the original default.
• Checked in headless Chromium against the built demo that ::part(e4) overrides the square colour, the custom colours and border apply, neighbouring squares are untouched, and the generated source block shows the CSS.
• pnpm typecheck, pnpm test (30 tests), pnpm build and pnpm build:pages pass.

Docs

readme.md gains a Styling section with both tables and examples. changelog.md gets an Unreleased entry.

Note: this PR and #24 both change the table builder in src/chess-board.ts and add an Unreleased heading to changelog.md, so whichever merges second needs a conflict resolution.

🤖 Generated with Claude Code

https://claude.ai/code/session_01VxKhiVNXdQ8Z1grVs38gY6

[github-actions]

Preview for 81d2b3c: https://0d31b68a.chess-board-4mh.pages.dev

Branch alias (always the latest push): https://claude-github-issues-triage-3zsy.chess-board-4mh.pages.dev