Add the Neutral template family, template picker and text-only logo allowances

Linea (the vendor's original format.pdf design), Monogram and Serenity are
rebuilt from the Canva PDFs as one parametric family on the engine, with
the user's logo in the measured slot, a home for every GST field and
A4/Letter support. Adds static Montserrat, Open Sans and Poppins fonts and
a generated Ms Madi outline for Serenity's vertical word. Text-only logo
placeholders (Monolith, Tangerine) use a doc-derived allowance box. A
Carbon TemplatePicker with committed thumbnails opens from the page
setup controls.

docs/templates/06-serenity.md

@@ -158,3 +158,20 @@ Implied margins: left 162.9 (rules) / 171.8 (text); right 64.1 / 70.8; top about
 - **Same internal grid as 4 and 5, shifted right:** rules 162.91-531.39 instead of 69.64-525.86 (left edge +93.3, right edge +5.5).
 - **Unique within the trio:** the paper texture, vertical script title, circular-text stamp, drawn signature, and the 'BANK DETAILS' / 'THANK YOU' bottom row.
 - **No resemblance to the in-app template.**
+
+## Implementation notes
+
+Built on the shared Neutral family (`src/pdf/templates/neutral/`, tokens in `serenity.ts`, decor in `src/pdf/decor/ScriptWord.tsx`). Measured against `6.pdf` with `npm run templates:compare` (`/tmp/invoice-c3/compare-serenity.png`).
+
+**Matches the design (measured, A4):** text column at x 171.8 and rules from x 162.91; Open Sans label and value widths agree to 0.1 pt; header baseline to first row 36.3 pt; paper #ECE8E1 as the Page `backgroundColor`; ink #3D3B3A; the vertical word and swash occupy Canva's band (word x 38.5-120, y 420-780; swash joined to the word's end and running up to y 143).
+
+**Intentional deviations, and why:**
+- **The word 'invoice' is outline art, not text.** Canva's SimpleSerenityScript is not shipped or traced. The word is drawn from Ms Madi Regular (SIL OFL 1.1; chosen over Sacramento because its hairline strokes are closer to Canva's) as SVG paths generated at build time by `scripts/decor/script-word.mjs` into `src/pdf/decor/scriptWord.generated.ts`, with the licence note in a comment. The font file stays in `scripts/decor/source/` only; it is never bundled, registered or embedded. The glyph contours are merged and shrunk by 0.95 pt per side (clipper-lib, a build-time dev dependency) to thin Ms Madi to a script-like weight; it is still heavier than Canva's. The swash is our own drawing (a tapered spine), not Canva's path, and the word is not tilted 1.1 degrees. The band is `fixed` page-level decor on every page, shifted up 32 pt on Letter. A real text 'TAX INVOICE' / 'INVOICE' label sits above the meta block, so the document is searchable.
+- **No paper texture.** Canva's JPEG is stock and is not bundled; the page is the flat measured colour #ECE8E1.
+- **Logo, not the stamp.** The user's logo goes in the measured stamp slot (82 x 82 pt, right edge on the text column). No logo, nothing: no ring, no stamp, no caption.
+- **Data text is tracked 0.05 em** (layout rule 7); labels and heads keep 0.10 em.
+- **Table.** The column is only 368 pt wide, so HSN/SAC is not a column: it is an 8 pt second line under each description that carries a code. Rate is amount plus short unit, and falls back to two lines in the 90 pt column when it does not fit.
+- **Totals.** The right-anchored column is 210 pt wide; the amount in words sits full width under it (the design's 200 pt left column does not exist at this width).
+- **Bottom row.** BANK DETAILS (inset 10.9 pt, as in the design), then THANK YOU above the signature image or the name, designation and computer-generated line; bottoms aligned. Canva's drawn signature is not reproduced.
+- **Margins.** Left 162.91 pt (never less, whatever the preset: the band is x 38-155), right 64.1, top 56, bottom 60 plus the 16 pt footer reserve. The header starts where the stamp slot does (y 131.75) and flows; without a logo the supplier block starts there instead.
+- **Elastic gaps:** top 75.75 to 40 pt, supplier-to-info down to 14 pt, info-to-table 61 to 28 pt; Letter also moves the band up.