[kimjooyoon]

Fixes #554.
Fixes #555.

Two compiler limit failures currently hide the useful diagnosis: a 256-upvalue count wraps and is accepted, while local/register overflow errors point at the enclosing function's start (line 0 for a main chunk).

This change:

• Rejects upvalue counts above 255 before narrowing to uint8, preserving the existing 255-capture capacity.
• Carries the declaration/header line into local-variable limit errors and retains the first source position that triggers register overflow accounting. The reported examples move from lines 0/5 to 202/206. Register limits, opcode accounting and error precedence remain unchanged.

Multiline declarations use their starting line; the AST does not retain individual name positions. The two issue fixes are separate commits in this PR.

Validation:

• Original master 75f497656b1c6864139dd2a7d88cf96d09550814 reproduces two cases where overflowing upvalue counts incorrectly compile and nine incorrect source-location cases. Five accepted-input controls remain valid.
• All 16 regression/control cases pass with both fixes together.
• Required make build passed; generated files remain unchanged.
• go test -p=2 -json -count=1 -timeout=90s ./... passed on Go 1.25.0 and 1.27.3, macOS/arm64, against the combined head.

AI assistance: OpenAI Codex (GPT-6) prepared the patches and tests; separate Codex agents reviewed them.