Raise instead of silent success: closed-channel send, missing load_file path, missing expression (#505, #490, #494)

[InauguralPhysicist]

Silent-tolerance audit batch-2d. Three paths that returned a silent wrong value with rc=0 now raise a catchable error from the closed error-kind set. Each was reproduced first, fixed, and given a regression test.

Fixes

• send: closed channel silently drops value (no error) #505 — send to a closed channel silently dropped the value. A producer that raced a consumer's close_channel lost messages with no signal. Now raises a catchable value error. Lock discipline: the enqueue path unlocks before returning; the closed path unlocks → val_decref → rt_error (never calls rt_error while holding the channel mutex). recv on a closed empty channel still returns null (EOF-like — intended).
• load_file: missing path returns null without raising (silent success) #490 — load_file of a missing/unreadable path returned a silent null (stderr warn + rc=0). Now raises a catchable io error, matching import's severity and the function's own parse/compile failure paths.
• parser: missing expression after is/of silently becomes null (x is, print of, throw of) #494 — parser folded a missing expression into a silent null. x is, print of, throw of, eval of "x is " bound/emitted null and continued with rc=0. parse_primary now raises Parse error line N: expected expression on a statement terminator (catchable parse via eval/import/load_file). Bare return — the one legitimate empty-expression position — checks the terminator itself and still yields null.

Tests

• test_raise_on_bad_input.eigs [116]: batch-2d cases + eval-catchable parse (count 43→48).
• New examples/errors/missing_expression.eigs [90] (direct-source rc=1).
• Docs: BUILTINS.md (send/load_file rows), CHANGELOG.md.

Validation

• Release suite: 2683/2683, 0 failed.
• ASan+UBSan with detect_leaks=1: 2683/2683, 0 failed, leak tally clean.

Closes #505
Closes #490
Closes #494

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