Expanded scripting/naming helpers: pad, sort_name/sort_initial, upper/lower_initial, acronym, validate_file_name, space/space_with, colon/colon_with

Ports the next slice of FileBot's ExpressionFormatMethods.java as Rhai-callable
methods usable inside any { } naming-scheme expression, alongside the existing
any/all_of/concat helpers:

- pad/pad_with: left-pad a value's string form (default '0'), non-mutating,
  non-truncating - deliberately a different Rhai method name for the 3-arg
  custom-char form (pad_with) since Rhai's own built-in pad(length, char)
  right-pads and mutates in place, which collides in behavior (not API) with
  what FileBot's zero-padded season/episode numbers need.
- sort_name/sort_initial: move a leading The/A/An article to the end for
  alphabetical sorting, e.g. "The Office" -> "Office, The".
- upper_initial/lower_initial: case-convert only the first character.
- acronym: first letter of each word, uppercased.
- validate_file_name: strip characters illegal in filenames, collapse
  whitespace runs, trim trailing dots.
- space/space_with, colon/colon_with: separator and colon normalization.

16 new unit tests in functions.rs (143 lib tests total, up from 126) plus 3
new integration tests in expression_format.rs demonstrating the helpers used
inside real naming-scheme expressions (14 total, up from 11). Verified via
cargo test --workspace, cargo check --workspace, and npm run build, all
clean. Updated README (status paragraph and porting-roadmap item 2).

README.md

@@ -64,7 +64,12 @@ file via a native Save dialog — this closes out the full original panel set (R
 Episodes/Subtitles/SFV/Filter/List); see "List" below. Real media-file probing (real
 resolution, frame rate, duration, and codecs read straight from a video file's own
 container header, not guessed from its filename) is wired into the Rename panel's
-"Match" step — see "Real media-file probing" below. See "Porting roadmap" below.
+"Match" step — see "Real media-file probing" below. The naming-expression engine's Rhai
+scripting layer also gained a batch of FileBot-style string helpers — `.pad()`,
+`.sort_name()`/`.sort_initial()`, `.upper_initial()`/`.lower_initial()`, `.acronym()`,
+`.validate_file_name()`, `.space()`/`.space_with()`, `.colon()`/`.colon_with()` — callable
+inside any `{ }` naming-scheme expression, e.g. `{n.sort_name()}/S{s.pad(2)}E{e.pad(2)}`;
+see "Porting roadmap" below.
 
 The naming scheme is fully user-editable from the app (the "Naming Scheme" button next
 to the New Names list) — the default is `{show name} - {season number}x{episode
@@ -593,9 +598,18 @@ ported from, for anyone picking this up:
    wiring probed bindings into the List/Filter panels (currently Rename-panel-only).
 2. **String/expression helper methods** — `net.filebot.format.ExpressionFormatMethods`
    (636 lines): FileBot-specific Groovy string extensions like `.pad()`, `.sortName()`.
-   Only a small subset (`any`, `all_of`, `concat`) is ported so far
-   (`crates/aurora-curator-core/src/format/functions.rs`); Rhai's built-ins cover some of the
-   rest already.
+   Ported so far (`crates/aurora-curator-core/src/format/functions.rs`): `any`, `all_of`,
+   `concat`, plus a newer batch of naming/scripting helpers — `pad`/`pad_with`
+   (left-pad, non-mutating; note this is a deliberately different name collision-avoidance
+   from Rhai's own built-in `pad(length, char)`, which right-pads and mutates in place —
+   not a fit for FileBot's zero-padded season/episode numbers), `sort_name`/`sort_initial`
+   (moves a leading "The"/"A"/"An" article to the end, e.g. `{n.sort_name()}` turns
+   `"The Office"` into `"Office, The"`), `upper_initial`/`lower_initial`, `acronym`,
+   `validate_file_name` (strips characters illegal in filenames, collapses whitespace,
+   trims trailing dots), and `space`/`space_with`/`colon`/`colon_with` (separator/colon
+   normalization). Rhai's built-ins cover some of the rest already. Still not ported:
+   FileBot's CSV/file-reading and shell-quoting helpers (`csv`, `read_lines`,
+   `quote_bash`, `quote_powershell`, etc.).
 3. **Matching/similarity engine** — `net.filebot.similarity.*`. Local filename-based
    season/episode detection is done (`crates/aurora-curator-core/src/matching`, ported from the
    pattern-matching half of `SeasonEpisodeMatcher.java`, adapted to Rust's `regex` crate