fix(driver): drop the unreachable retained-source validation fallback

[e54-bot]

Summary

Removes the #583 retained-source recovery fallback in session.rs, which #626 shows is unreachable after the workshop-rs 1.13 bump (#624/#625): set_file_source prunes provenance spans that do not resolve in the attached document, and the only source-dependent validate() check (wir/validate.rs byte_range) runs on spans pruning already removed. A remaining validate() failure is therefore source-independent — the unretained re-parse could only re-surface the same error, never recover.

• deleted: the unretained re-parse, map.apply re-application, retained_source flag, and the source-map-span-overflow warning
• validation errors now return via workshop_diag_for_provider_artifact directly
• the span-out-of-range regression test keeps its assertion (Mapped provenance still survives) and its doc gains Remove the now-unreachable retained-source validation fallback in session.rs #626

Test plan

• cargo test -p wright-driver --test source_provider — 37/37, including mapped_span_outside_retained_source_degrades_extents_not_the_load
• cargo fmt --all -- --check, git diff --check
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo test --workspace --all-targets --all-features — full workspace green
• git grep source-map-span-overflow — no remaining references

Fixes #626

[e54-bot]

Independent review — LGTM. Verified unreachability end to end: SourceMap::apply rebinds the file table, bounds/validates all mapped spans, and settings.clear_spans() covers the non-provenance spans; set_file_source's prune_spans_outside covers every provenance slot; to_wir sources WIR spans only from provenance; check_span's byte_range is the only source-dependent check and runs only for files with attached source, where post-prune all spans resolve. The !retained_source early-return is equivalent to the new error path (same &source_files-iff-Mapped argument). No dangling source-map-span-overflow references, no dead imports; test comment accurate.