Append duplicate parameter values without copying prior arrays

[OskarEichler]

Summary

Append duplicate parameter values to their existing array instead of allocating and copying the accumulated array on every occurrence.

Reproduction and verification

p = WebSocket::Extensions::Parser
p p.parse_header('ext; v=1; v=2; v=3').by_name('ext')
# [{"v"=>[1, 2, 3]}]

1,000 ordered duplicate values verified. In a paired cumulative-parser benchmark, a 1,000-parameter header fell from about 8,013 to 5,017 allocations and 0.961 to 0.594 ms per parse. The exact timings include other verified parser corrections; the isolated source change removes repeated prior-array copies.

• Ruby 4.0.6 through rbenv; existing current-upstream suite: 64 examples, zero failures on this isolated change.
• The cumulative release-based candidate passes 283 focused checks (272 baseline failures → zero), 2,784 model checks, the current upstream's 64-example suite, and gem build/extraction.
• The historical 0.1.5 suite has three Ruby keyword-versus-options-hash mock failures on both baseline and candidate. Current upstream already corrected those expectations; they were run against the candidate through an external preload without changing repository tests.
• No new or modified tests/specs, following the consumer repository's explicit policy. Reproductions/models were run from external scratch scripts.

Breaking changes and limitations

No intended breaking change. Parameter order, values and single-value representation are unchanged; arrays are owned by this parse, not supplied by callers.

Only local Ruby 4.0.6/macOS execution is claimed; the repository's older Ruby/JRuby matrix needs upstream CI. No production access or unrelated release upgrades.