fix: report bare identifier assignment as a compile error

[otnc]

Summary

x be value on a bare identifier was parsed permissively and the assignment silently dropped — it compiled to x;, so code ran with stale values and no diagnostics.

Per the RFC (6.4 Assignment), bare identifier assignment is not supported by design (it prevents accidental implicit globals). This PR makes the compiler actually say so instead of silently discarding the code.

Changes

• parser: new Parser::errors array collects diagnostics; bare identifier assignment pushes a descriptive error:

  Error: bare identifier assignment ('y be ...') is not supported. Declare with 'const y be ...' or 'let y be ...', or use compound assignment such as 'y add be 1'
  

• cmd: purus build / purus check print collected errors and exit 1
• pkg: compile() / check() now surface the compiler's output in the thrown Error instead of a generic "Command failed" message

Property (obj.x be), index (arr[\0] be), slice, destructuring ([a; b] be), and compound (x add be 1) assignments are unaffected.

Tests

• moon test --target js: 320 passed
• Added parser tests: error recorded for x be 10 and for reassignment after let; no errors for property/index/compound assignments
• E2E: compile('let y be 10\ny be 20') now throws with the message above; valid code still compiles

Note for docs

The website docs currently contradict the RFC on this point (operators.mdx shows y be 20; control-flow.mdx uses i be i add 1 in while examples — those examples infinite-loop today). They should be updated to use compound assignment; I can follow up with a puruslang/docs PR.

Fixes #5