Perf/throughput pass (5/8)

[bnomei]

Linked Issue

Closes #

Description

Improve encoder and decoder throughput by reducing allocations and adding fast paths without changing behavior. This caches indentation, streams quoting, uses canonical number formatting, and adds ASCII fast paths in scanning while reducing allocations in parsing and expansion. Preallocation is used where sizes are known, and field-list validation is linearized. This is PR 5 of 8 in the intended sequence (1-8).

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)
• Performance improvement
• Test coverage improvement

Changes Made

• Cache indentation and streamline quoting and number formatting in the encoder.
• Add ASCII fast paths and reduce allocations in scanner and parsing.
• Preallocate arrays/rows/buffers and streamline field-list validation.

SPEC Compliance

• This PR implements/fixes spec compliance
• Spec section(s) affected:
• Spec version:

Testing

• All existing tests pass
• Added new tests for changes
• Tests cover edge cases and spec compliance

Tests run: cargo test
Benchmarks run: cargo bench --bench encode_decode -- --baseline current --noplot

Pre-submission Checklist

• My code follows the project's coding standards
• I have run code formatting/linting tools
• I have added tests that prove my fix/feature works
• New and existing tests pass locally
• I have updated documentation if needed
• I have reviewed the TOON specification for relevant sections

Breaking Changes

• No breaking changes
• Breaking changes (describe migration path below)

Migration path:

Additional Context

Branch: perf/throughput-pass-1
Intended PR order: 1-8 (this is PR 5 of 8)