v1 Milestone - Complete single file language

[jackwthake]

Reaches the v1.0 milestone from docs/roadmap.md: every must-have and
should-have feature for a "complete single-file language" is implemented and
tested. Only explicitly-optional nice-to-haves remain (short-circuit &&/||
outside boolean context, &=/|=/^=/<<=/>>= compound assignment).

Language features

• break / continue for while and for loops, with analyzer checks
  rejecting either outside a loop.
• Local string literal initializers: u8 *p = "..."; now works inside
  function bodies, not just at global scope.
• 16-bit multiply/divide/modulo (__mul16, __div16, __sdiv16).
• __heap_start / __memory_top implicit compiler globals for bump
  allocators and RAM-top queries.
• Embedded ROM symbol table ("C02S") so c02-objdump resolves real
  function names instead of auto-generated labels; --strip-debug to omit it.

Correctness fixes

• Variable shadowing is now a semantic error (ERR_SHADOWED_DECLARATION).
  Root cause: codegen identifies variables by bare name with no scope
  qualifier, so a shadowed inner declaration silently aliased its outer
  namesake's storage — this caused break to fire on the wrong loop
  iteration. Shadowing an enclosing scope is now rejected at analysis time;
  reuse across non-overlapping sibling scopes is unaffected.
• Signed 8-bit div/mod (__sdiv8), i8 division/modulo was routing
  through the unsigned helper and producing wrong results for negative
  operands.
• Binary op widening/signedness: mixed-width and mixed-sign binary ops
  derived type/signedness from the left operand only, causing dropped high
  bytes (u8 + u16) and trichotomy violations in comparisons (i8 < u8 vs
  u8 < i8 disagreeing). Both operands now normalize to a common type before
  the op.

Safety / robustness

• ROM overflow detection: all ROM writes are now bounds-checked; a
  program exceeding 32 KB fails with a diagnostic instead of silently
  corrupting the output.
• ZP map overflow propagation: zero-page exhaustion now fails codegen
  cleanly instead of printing a warning and continuing.