Batch rename for the §5.1 generic filenames, paired with their prose edits

This is the dedicated issue that decision D6 in QuantEcon/workspace-lectures#39 promised ("a batch rename across all five §5.1 files gets its own issue") and that the QuantEcon/workspace-lectures#42 validation confirmed had still not been filed after three separate promises. The register entry at #66 (decision 5, "the rename list for generic filenames") carries the same list; this issue is where the work gets planned and tracked.

The published tree is flat, so the key is a bare filename. From .dev/qeld/migration-catalog.md §5.1, the five names and where each now stands:

Two constraints the catalog already records:

• A rename breaks text qeld cannot reach. The catalog's example is mle.md's prose naming mle/fp.dta; every rename must be paired with its prose/{download} edits in the same PR, found by grep rather than by the audit (§5.2 lists 28 prose refs, 11 naming a data file in two places).
• Timing: the catalog wants this pass done before the remaining tracks migrate — that now means before Tracks C and D land their files, so the three unmigrated names arrive under their final names rather than needing a second breaking change.

For fp.dta, renaming now means a breaking URL change and a coordinated repoint wave for one file. The cheap alternative is to fold it into the Phase 4 interim→final cutover (QuantEcon/data-lectures#15), where every consumer URL changes anyway — proposed default, decide here.

Start with .dev/qeld/migration-catalog.md §5.1 and the prose references listed in §5.2; use grep to find filenames and {download} links, then review QuantEcon/data-lectures#15 for the fp.dta timing. Done means deciding replacement names, renaming the three pending files, fixing the dangling data.csv reference, and updating every affected prose reference.