Stream rewritten SPDX identifiers

[andrew]

RewriteIdentifiers currently builds a second expression tree and then renders it. This change writes rewritten identifiers directly to a strings.Builder while preserving callback order, precedence, license references, exceptions, special values, and input immutability.

For an expression with 128 identifiers, the local benchmark falls from about 20.5 microseconds and 71.6 KB allocated to 4.7 microseconds and 3.0 KB. A differential fuzzer compares the streaming result with the former clone-based implementation across valid syntax trees.

[Copilot]

🟢 Approval recommended

The streaming implementation matches existing String() precedence/parenthesization rules (including WITH wrapping under OR), preserves callback order, and is backed by differential fuzzing plus existing rewrite-order tests.

Pull request overview

This pull request optimizes RewriteIdentifiers by streaming rewritten SPDX expression output directly into a strings.Builder instead of cloning and re-rendering a second expression tree, while preserving the existing normalization/precedence rules and rewrite callback order.

Changes:

• Replace clone-based rewriting with a streaming writeRewritten(*strings.Builder, rewrite, parentToken) implementation across expression node types.
• Add a fuzz test to differential-check streaming output vs a clone-based reference implementation, plus a size-scaled benchmark.
• Update the Expression interface to support streaming rewriting.

File summaries

File	Description
rewrite.go	Implements streaming rewrite via writeRewritten methods and a small fast-path in RewriteIdentifiers.
rewrite_stream_test.go	Adds differential fuzzing against a clone-based implementation and benchmarks across expression sizes.
parse.go	Updates Expression interface to include writeRewritten, enabling streaming rewriting across node types.

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 0
• Review effort level: Lite

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