Add etchy-kicad S-expression parser

[Cimos]

Phase 1, PR 3 of the native KiCad plan.

• New crate crates/etchy-kicad, no external dependencies, builds for wasm32.
• Byte-level lexer and node tree: lists, atoms, quoted strings with KiCad escapes, numbers kept as source text, byte/line/column positions on every node.
• Every node keeps an id; Consumption lets later decoders mark what they used and list unconsumed nodes by path, for the record inventory in PR 4.
• Limits for native and wasm (input bytes, nesting depth, node count, atom/string length), each a typed error with its position.
• Typed errors for invalid UTF-8, unterminated strings, bad escapes, unbalanced parens, empty input, trailing data and numeric traps; checked helpers for integers and millimetre-to-nanometre conversion.
• Reads the root and header (version, generator, generator_version) only.

Tested: 14 unit tests including mutation/truncation never panicking. The public Mad_RP2040 board (KiCad 9, 4 MB, 599,750 nodes) parses in about 90 ms in release; an optional smoke test runs it when ETCHY_KICAD_SMOKE_FILE is set at build time. The first version re-checked the rest of the file for every string character (17.5 s on that board); fixed, with a linear-time regression test.

Refs #122