translation/13: Blocks language — CSP-aligned procedural subset

[ChrisMayfield]

Brings the visual Blocks (Blockly) language up to a coherent, well-tested feature set. Rather than chasing full parity with the typed/OOP languages, Blocks now implements a deliberate subset of the Universal AST — the same procedural, untyped, non-OOP surface CSP exposes — because its audience is middle-school programming and AP CSP. Everything outside the subset simply has no block, so a student can't build a program that fails to run or translate (mirroring how CSP marks OOP unsupported).

Blocks feed the shared interpreter/translator, so this touches only src/language/blocks/*, tests, docs, and a sample — no changes to interpreter.ts/translator.ts or any text parser.

What's added

• Loops: repeat forever (→ While(true)), for each … in a list, and recognition of the counting-for pattern AP CSP's REPEAT n TIMES lowers to.
• Lists (1-based): literal (stock lists_create_with), get/set item, append/insert/remove, length.
• Strings (1-based): substring, character-at, upper/lower case, contains.
• Math / random / conversion: abs, sqrt, min, max; random / random-int / random-range (RANDOM) / seed; to integer/decimal/text.
• Print: a terminator dropdown (newline / space / none) so Print.appendLineFeed and separator round-trip faithfully — notably CSP DISPLAY's trailing space.
• Toolbox & theme: reorganized into Common / Logic / Loops / Math / Text / Lists / Variables / Functions, each with a distinct legible hue on the dark slate theme.
• Docs & sample: specs/blocks.md (subset + intentional omissions), examples/demo.blocks.json, and README updates.

Indexing is 1-based in the UI and folded to the 0-based AST in fromAst/toAst, exactly as csp/parser.ts ↔ csp/emitter.ts do.

Intentional omissions (converters reject them): classes/OOP, typed declarations, try/catch, switch, ternary, ++/--, +=, C-style for, do-while, null.

Testing (Option B)

Fidelity is verified by interpreting: a program must produce the same output run directly and after AST → blocks JSON → real headless Blockly load/save → AST. The whole demo.csp passes this, alongside focused per-feature programs and a load+interpret check of demo.blocks.json. blocks.test.ts: 39 tests.

• npx tsc --noEmit, npm run test:run → 508 passed
• npm run build succeeds; dev server serves the editor
• npm run test-browser (Selenium csv/) → 90 passed, 0 failed (text languages unaffected)

Two deviations from the plan

• Kept praxly_repeat_until (post-condition) rather than retiring it — it round-trips cleanly and the pre-condition controls_whileUntil (UNTIL) already covers the CSP/Scratch "repeat until".
• Deferred indexOf — its returned-position base convention isn't fixed by the CSP reference; noted in specs/blocks.md.

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