Reduce allocations in MakePURLString

[andrew]

Route MakePURLString through the direct package URL encoder and pre-size output buffers using escaped lengths. This avoids intermediate PURL allocations while preserving canonical component escaping across ecosystems.

Add benchmarks covering package and version PURL construction for npm, scoped npm, and Go, plus compatibility cases for ecosystem-specific splitting and escaping.

[Copilot]

Pull request overview

This PR optimizes PURL string construction by routing MakePURLString/BuildPURLString through a direct encoder path and pre-sizing the output buffer based on escaped component lengths, reducing intermediate allocations while preserving canonical escaping behavior across ecosystems.

Changes:

• Introduces a shared buildPURLString fast-path encoder with capacity pre-sizing based on escaped lengths.
• Refactors ecosystem string handling toward shared constants and updates MakePURLString to use the direct encoder.
• Adds expanded MakePURLString compatibility tests and new benchmarks covering package/version PURL construction for npm (including scoped), and Go.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
makepurl.go	Adds direct PURL string builder + escaped-length estimators and updates namespace encoding loop.
ecosystem.go	Centralizes ecosystem constants and routes MakePURLString through the new builder.
ecosystem_test.go	Expands MakePURLString tests to cover escaping and ecosystem-specific behavior with canonical cross-checks.
ecosystem_bench_test.go	Adds benchmarks for MakePURLString across representative ecosystems/packages.

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