docs: correct early-exit ITGL backlog details

[SDL-HQ]

Motivation

• Clarify the Evidence hardening backlog entry so it no longer states mixed-mode failures reuse generic ITGL components (mixed-mode now has a dedicated mixed_mode_validation component) and accurately describes which early-exit paths remain generic.

Description

• Replace the Notes cell for "Granular early-exit ITGL components" in docs/backlog.md to: "ISC structure and structured schema declaration failures produce ITGL entries that reuse generic component types, with the specific error code carried in entry data. The malformed payload early return produces no dedicated ITGL entry. Dedicated component types per error class are not implemented for these paths." and do not modify any other row or file.

Testing

• Ran pytest -q which passed (139 passed), and confirmed docs/backlog.md is the only file changed and the working tree is clean.

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Codex Task