Add exact text semantic reference and oracle

[isomorphisms]

Summary

• add a deliberately simple Idriç exact-substring semantic reference
• expose contains_string, overlapping zero-based character match_positions, and match_count
• add deterministic fixtures for absent/beginning/middle/end, overlap, one-byte, repeated-prefix, and empty-pattern behavior
• add a focused CI job compiling/running the oracle with IB's pinned Idriç compiler
• document the distinction from PR Add filesystem Float32 vector backend #17's Float32 vector exact scan and the intended ARM/Thumb optimization boundary

Why this slice first

PR #9 described a future grep-like extraction pass but did not implement one. This makes that ordinary-text operation real without prematurely baking an ARM trick into browser semantics.

The current ARM/Thumb backend is still straight-line and numerical; PR #8 is a branch/dispatch acceptance-spec change, not string/loop lowering. The browser oracle therefore lands as a separate slice. Target-specific work remains tracked in isomorphisms/Idric#21 and isomorphisms/idric-arm-thumb#9, #10, and #11.

Semantics

• case-sensitive Char substring matching
• overlaps count
• positions are zero-based character positions
• empty pattern matches every character boundary, including the end

The first ARM comparison should use ASCII fixtures so byte and character offsets coincide. Any later Unicode-optimized API must state its offset units explicitly.

Refs #33.