fix: correct the license to PIF and ship a README with the crate

[spacedevin]

Two things wrong on the published deckfile crate page.

License was MIT — should be PIF

Pay It Forward, the same license as tish. LICENSE now carries the PIF text (with this project's own copyright line) and package.json says PIF.

The crate manifest uses license-file rather than license, because PIF isn't an SPDX identifier — an SPDX field would either be rejected by crates.io or misreport the terms. That's what tish does, for the same reason.

No README at all

deckfile v1.2.1 published with an empty crate page: "deckfile v1.2.3 appears to have no README.md file". crates.io renders the page from a README inside the package and can't reach back into the repo, and build-rust.mjs never generated one.

Both LICENSE and README.md now ship — 32 files packaged, up from 30.

The README is written for a Rust consumer rather than reusing the npm one, since they need different things:

• leads with parse() and the typed BodyRow match, not npm install and the Tish import path
• notes that parse never fails — malformed lines land in program.errors, because a streaming host has to parse partial programs
• says plainly that the crate is generated from src/index.tish, that Rust/JS/Tish therefore agree by construction rather than by discipline, and that patches belong upstream
• states the scope boundary (no apply/emit/audio) and the registry-based extension points

Verification

npm test                 ALL_OK · CONFORMANCE_OK (11 cases) · tish smoke all passed
cargo test (crate)       4 passed
cargo publish --dry-run  Packaged 32 files, 42.2 KiB compressed
cargo package --list     LICENSE, README.md both present

Takes effect on the next release; 1.2.3 keeps the blank page and MIT metadata until then.