[EtaCassiopeia]

Closes #338

Why

rift 0.24.0 (rift#1566) accepts a multi-character csv.delimiter ("||"), as Mountebank does. LookupSpec.fromCsv only took a char.

What

• New LookupSpec.fromCsv(String path, String keyColumn, String delimiter). The value is sent verbatim.
• The char overload is kept (source and binary compatible) and delegates to it, so its wire output is byte-identical to before.
• Refused up front:
  • null → NPE;
  • "" → IAE (the engine reads it as ,, which fromCsv(path, keyColumn) already means);
  • an unpaired surrogate anywhere in the string → IAE (it would reach the engine as ?).
  • A properly paired supplementary character is accepted.
• The Optional<Character> field becomes Optional<String> (private).
• Docs: javadoc, and docs/design/sdk-api.md in the lookup grammar block.

Tests

• LookupSpecWireTest: +6 tests (14 total):
  • "||" gives the literal wire JSON;
  • ";" and ';' produce identical output;
  • empty and null are refused;
  • an unpaired surrogate is refused;
  • an emoji is accepted.
• LookupGrammarIT: a new live case reads a ||-delimited CSV and serves the looked-up row.
• Local runs: full verify on engine v0.24.0 passed on SPAWN and EMBEDDED.

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