seconv: --multiple-replace accepts GUI-exported .template JSON and .csv

[niksedk]

Fixes #12544.

Problem

In SE4 the CLI (SubtitleEdit.exe /convert) shared the GUI's Settings.xml, so multiple-replace rules were automatically the same in both. SE5 broke that link: seconv --multiple-replace reads only the legacy MultipleSearchAndReplaceGroups XML, but the SE5 GUI (Tools → Multiple replace → export) writes .template (JSON) or .csv — never that XML. So there was no way to use GUI rules in seconv, which is exactly the dead end the reporter hit. (--settings can't help either: the rules live in the UI settings tree, not libse Configuration, so that path never carries them.)

Fix

MultipleReplaceLoader now detects the format — by extension, then by sniffing the first non-whitespace char (< = XML, { = JSON, else CSV) — and loads all three into the same rule list:

• XML — unchanged.
• JSON — the GUI's CategoryImportExportItem shape (categories[].rules[].{find, replaceWith, description, isActive, type}), read camelCase-insensitively.
• CSV — the GUI's exact columns (Category,Find,ReplaceWith,Description,Active,Type) with RFC 4180 quoting; the parser is ported verbatim from the UI's CsvImporter so round-trips are guaranteed.

The UI's MultipleReplaceType names map to the XML SearchType the apply loop expects (CaseInsensitive → Normal; CaseSensitive/RegularExpression by name). Only active rules are applied. Existing XML behavior is untouched.

Docs

docs/reference/command-line.md now leads with the GUI-export workflow (export as .csv/.template, pass straight to --multiple-replace), shows the CSV shape, and keeps the legacy XML sample.

Verified

• 4 unit tests: XML, CSV and JSON produce byte-identical replacements — covering CaseInsensitive/CaseSensitive/RegularExpression, an inactive rule that must be ignored, a case-sensitive non-match, and extension-less content sniffing.
• seconv end-to-end: the same rules as .xml, .csv and .template all yield identical SRT output.
• All 237 seconv tests pass.

Change-log updated (also folds in the other rc5 entries merged since the last changelog update: EBU-TT-D, the F10 fix, the culture-parse fix, and the perf rounds).

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