Reduce URL percent-encoding allocations

[quinnj]

The shared URL encoder formats and uppercases a temporary string for every escaped byte. Write each percent triplet directly from a hexadecimal digit table instead. This preserves the encoded bytes and existing unreserved-character rules while reducing allocations on every path/query parameter that requires escaping.

Warmed Julia 1.12.6 measurements on Apple Silicon (median of seven batches of 10,000 calls):

Input	Before	After	Allocated bytes before → after
128 spaces	14.72 μs	2.55 μs	44,000 → 992
Unicode/path/query text	1.98 μs	0.54 μs	5,088 → 384
hello world!	352 ns	160 ns	864 → 192
Unescaped ASCII	193 ns	194 ns	192 → 192

Validation: full suites pass on Julia 1.10.11 and 1.12.6, including generated HTTP client/server tests, the JuliaC trimmed executable, and the pinned Petstore/Discord corpus. New checks cover all 256 byte values, Unicode, substring offsets, unreserved characters, and percent signs. git diff --check passes. No API or generated-runtime contract changes.

Co-authored by Codex