M2.2: Add static-only QPACK encoder

[RomanEmreis]

Summary

Implement a minimal static-only QPACK encoder in istok-core/src/qpack/encoder.rs.

The encoder must produce valid QPACK header blocks using:

• Indexed Field Line for exact static-table matches
• Literal With Static Name Reference for name-only hits
• Literal Without Name Reference otherwise

This milestone is static-table only:

• Required Insert Count = 0
• Delta Base = 0
• no dynamic table
• no Huffman

Scope

• add encoder.rs
• encode a slice of HeaderField pairs into a QPACK header block
• emit the required prefix:
  • RIC = 0
  • S = 0 / Delta Base = 0
• choose field line representation according to static-table-first rules
• preserve field order
• remain allocation-free and write into caller-supplied &mut [u8]

Acceptance criteria

• encoding [(:status, "200")] produces an Indexed Field Line using static index 25
• encoding [(:status, "999")] produces Literal With Static Name Reference
• encoding [("x-custom", "val")] produces Literal Without Name Reference
• encoding multiple fields preserves order
• encoded output always starts with the two zero-byte prefixes
• returns BufferTooSmall when the output buffer cannot fit the block
• roundtrip with decoder (M2.3) recovers the original fields

DoD

• all acceptance tests green
• clippy clean
• milestones.md updated