Add edit mode for data sets/members: e to edit, explicit save only (ctrl+s)

[Tannex]

Closes #17. Stacked on #24 (issue-16-dataset-favorites); merge that first — this PR's own changes are the last commit.

What

• internal/zosmf: new opt-in TextEditor interface (ReadText/WriteText) implemented by Client and LazyClient. ReadText requests X-IBM-Return-Etag and captures the ETag; WriteText sends a text-mode PUT with If-Match so a host-side change fails the save (IsConflict detects HTTP 412). The read-only Browser interface is deliberately unchanged — cqt type-asserts for the write surface and degrades gracefully.
• internal/cqt: e on a PS data set (dataset list) or PDS member (member list) fetches text and opens a full-area textarea editor with EDIT <target> chrome and a MODIFIED indicator; enter behavior is unchanged. RECFM U (load libraries) and unsupported DSORGs get a status message instead of an editor. ctrl+s is the only write path: it validates every line against LRECL (minus the RDW for V formats) and rejects the save naming the offending line. Conflicts keep the buffer and offer ctrl+r (explicit reload). esc on a dirty buffer asks for confirmation (d discards, esc/any key keeps editing); clean exit is immediate. Inside the editor q types into the buffer; ctrl+c still quits.
• cmd/cqt: --read-only flag disables e entirely; demo mode implements the full edit/save/conflict flow in memory (shared across demo profiles).
• README: keys table, synopsis, and a new Edit mode section (including the stty -ixon note for terminals that reserve ctrl+s).

Decisions on the issue's open questions

• LRECL overflow: reject on save, pointing at the offending line (no truncation).
• e from the records view: out of scope for v1 — lists only.
• ctrl+s vs XOFF: kept ctrl+s; documented stty -ixon in the README.
• Encoding: write-back uses z/OSMF text mode (same conversion contract as the existing FetchText read path); the buffer round-trips byte-identically with a single trailing newline normalized.
• If the host returns no ETag on fetch, the editor warns that saves cannot detect concurrent changes and omits If-Match rather than blocking editing.

Validation

go build ./... && go vet ./... && go test ./... all pass. New tests: 6 in internal/zosmf (conditional GET/PUT headers, ETag round-trip, 412 conflict detection, target normalization) and 9 in internal/cqt (open/eligibility, dirty tracking, save gating, LRECL rejection, conflict + reload, discard confirmation, key routing, --read-only, missing write surface), plus demo round-trip/conflict tests in cmd/cqt.

🤖 Generated with Claude Code

Review follow-up

• Encoding: text-mode payloads are ISO 8859-1 on the wire; the editor now decodes them to UTF-8 on fetch and encodes back on save, so ÆØÅ round-trips. LRECL validation measures encoded host bytes.
• Line numbers: fixed five-column gutter.
• Scroll inertia: profiled to ~19 ms per keypress at 1000 lines (the textarea re-renders its whole buffer per update, slower than key repeat, so events queued and drained after release). Held navigation keys are now coalesced into one batch per 12 ms tick.
• ctrl+c: removed inside the editor (and the jq popup); esc with the dirty confirm is the way out.

[Tannex]

Edit mode does not seem to map encoding very well, ÆØÅ renders fine in view, but not in edit.

[Tannex]

Always reserve 5 characters for line number

[Tannex]

Holding arrow down in edit mode will keep scrolling for a bit after it is released

[Tannex]

Get rid of explicit ctrl+c binding in edit mode.