fix: missing filename header when scrolling up in live_grep (#470)

[gustav-fff]

Closes #470

Root cause

lua/fff/grep/grep_renderer.lua:207 — file grouping logic uses ctx._grep_last_file to track consecutive matches from same file. This state resets per render (context is fresh each call). When user scrolls up past page boundary and new results load, first item in new page compared against nil → looks like different file but no way to distinguish "truly new file" from "first visible item after pagination". Result: no header rendered even though file might be same as last item on previous page.

Fix

Always render header for first visible item (ctx.iter_start). Ensures filename always shown at top of viewport regardless of pagination state. Trade-off: duplicate header if scrolling within same file, but matches user expectation — "I see a match, I need to know which file".

Steps to reproduce

Requires large repo with multi-file grep results spanning multiple pages:

```sh
cd # e.g. linux kernel, chromium, large monorepo
nvim
:lua require('fff').search('function') -- common keyword with many hits

Navigate down past first page boundary (e.g. j × 100)

Navigate back up past page boundary (e.g. k × 100)

Observe: top filename header missing on scrolled-to page

```

Expected: filename header visible for topmost match.
Actual (pre-fix): filename missing, only :line:col and content shown.

How verified

Code inspection: change targets exact root cause. Logic: is_first_visible = (item_idx == ctx.iter_start) ensures first rendered item always gets header. Checked signature match with file_renderer.render_line (third param already present but unused). Ran formatter (stylua passed).

Automated triage via gustav-fff bot.

[dmtrKovalenko]

@gustav-fff rebase and retest thsi

[gustav-fff]

[triage-bot] DIRECTED: rebased onto main (3a465e8).

Conflict in lua/fff/picker_ui/grep_renderer.lua — main moved the file from lua/fff/grep/ and kept render_line(item, ctx) (no item_idx param). Resolved by tracking first-rendered via ctx._grep_first_rendered flag instead of comparing against ctx.iter_start. Same behavior, no signature change, list_renderer caller untouched.

CI will retest.

Honk-Honk 🪿

[dmtrKovalenko]

why this is unused?

keep this used and add this to the lua defintion params

cc @gustav-fff work on this PR and babysit it

[gustav-fff]

[triage-bot] DIRECTED: pushed fbce94b.

render_line(item, ctx, item_idx) — list_renderer now passes i consistently with apply_highlights. First-visible check is item_idx == ctx.iter_start, no stateful flag. file_renderer.render_line signature updated to accept the optional param.

Honk-Honk 🪿

[dmtrKovalenko]

@gustav-fff _grep_last_file -> rename this to gre_last_file as it is now used

AND ACUTALLY WAIT FOR CI TO PASS BEFORE LEAVING THIS PR

[gustav-fff]

[triage-bot] DIRECTED: pushed 369a6a6 — renamed ctx._grep_last_file to ctx.grep_last_file (drop underscore prefix since the field is part of the render context now).

CI kicked off, currently pending across the build matrix. Will not consider this done until it goes green.

Honk-Honk 🪿

[dmtrKovalenko]

@gustav-fff fix the luacheck lint

[gustav-fff]

[triage-bot] DIRECTED: pushed ab5301c.

lua/fff/picker_ui/file_renderer.lua:23 — added -- luacheck: ignore item_idx on the signature. The param is intentionally unused in file_renderer (only grep_renderer consumes it), keeping the shared render_line(item, ctx, item_idx) shape that list_renderer calls through.

Honk-Honk 🪿