fix: correct numeric string coercion with leading zeros

[ljluestc]

What problem does this PR solve?

Issue Number: close #478
L.DoString returns an incorrect result for string-to-number coercion in arithmetic expressions.

Repro script:

a = "00001" + "00011"
print(a)

Expected: 12
Actual: 10

This is inconsistent with Lua 5.1 arithmetic coercion expectations for numeric strings.

What changed and how does it work?

This PR updates numeric coercion during arithmetic evaluation for string operands:

1. Parse decimal integer strings using base 10 first, so leading-zero values are treated as decimal (not octal).
2. Preserve hex-prefixed parsing behavior for values like 0x7d6.
3. Keep existing fallback behavior for floating-point parsing and non-numeric failures.
4. Add regression coverage in both Go tests and _glua-tests/issues.lua.

Why this approach?

• Aligns runtime behavior with Lua 5.1 coercion semantics.
• Fixes the reported user-visible bug with a narrowly scoped change.
• Adds regression coverage to prevent future breakage.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
  • Executed:
    • go test -run 'TestDoStringNumericString(Coercion|HexCoercion)$' -count=1
    • go test -run TestGlua -count=1
  • Covered repro:
    • a = "00001" + "00011" -> 12
• No need to test

Documentation

• Affects user behaviors
• Additional docs required

Release note

Fix incorrect string numeric coercion in arithmetic so expressions like "00001" + "00011" evaluate correctly.