playground: showcase the features it has not heard of, and say which compiler it is

[simontreanor]

The playground has not been touched since it shipped in #2. Everything from 0.2.0 through 0.5.0 landed after that, and the docs were updated for it while the playground was not, so the picker advertises the language as of 0.1.0.

The gap that matters most: computation expressions are one of the two F# showcase features this project leads with, and they had no playground presence at all. Opaque types (0.2.0) and destructuring let (0.5.0) were likewise absent.

Two new examples

Both were extracted from app.js and run through the real compiler, along with the five existing ones (all seven still compile and run):

#	Example	Output
1	Computation expressions — option { }, the result { } it mirrors, and seq { }	Some('Ada is 36') / None_ / Ok(7) / Error('not a number') / [1, 2, 3]
2	Opaque types — the zero-cost newtype, plus destructuring let including the record form	u-1001 / credits between 7 and 17

Which compiler is this?

The page never said, so a stale deploy was invisible and a visitor could not tell what they were trying. pyfun::VERSION is now exported from the library (the binary and the LSP each had their own env! call), the WASM shim re-exports it as version(), and the footer shows it. The footer also links back to the course, which every page of the course links into and nothing linked back from.

Two real bugs

• Diagnostics drifted on non-ASCII source. Spans from Rust are byte offsets; lineCol walked UTF-16 code units, so a single accented character earlier in the buffer moved the reported line:col of every later diagnostic. It now walks the encoded bytes, counting characters for the column.
• Two internals-tour blocks always failed their Run button. docs-run.js attaches Run to every code.language-pyfun block, and these two were fragments quoted mid-prose with no type Shape / measure m in scope, so clicking Run always produced unknown constructor / unknown measure. Both are now complete programs. That also fixes the exhaustiveness chapter, which quotes a diagnostic at --> 4:3 above a three-line block: with the type declaration restored, line 4 is where the match actually is.

Also

The docs highlighter (docs/theme/highlight.js) says in its own header that it mirrors editors/vscode/pyfun.tmLanguage.json. It had drifted from both that and the lexer: no option, no opaque, no as, and it highlighted in, which is not in Tok::keyword at all. Lesson 13's option { } and lesson 22's opaque type were rendering unhighlighted on the live site.

CI: the Pages build installed whatever wasm-pack was newest that day (the one unpinned tool in a pipeline that otherwise reproduces exactly) and recompiled the whole compiler for wasm32 from scratch on every deploy. Now pinned to 0.15.0, matching the local toolchain, and cached. Pyodide goes to 314.0.3.

Verification

wasm-pack build succeeds and exports version; node --check passes on all three playground modules; every bundled example runs; cargo test, cargo clippy --all-targets -- -D warnings and cargo fmt --check are clean. I could not click through the rendered page (no browser extension connected here), so the footer version string and picker label are verified by construction rather than by eye.