Add 27 new page size constants to both domain and creator layers:
- Extended ISO A series: A0, A1, A2, A6, A7, A8
- Extended ISO B series: B0, B1, B2, B3, B6
- ISO C/Envelope: C4, C5, C6, DL
- North American: Executive, HalfLetter
- ANSI Engineering: ANSIC, ANSID, ANSIE
- Photo: Photo4x6, Photo5x7, Photo8x10
- Book Publishing: Digest, USTradeBook
- Presentation (unique): Slide16x9, Slide4x3
- US Envelope: Envelope10
- JIS B (Japanese): JISB4, JISB5

Domain layer uses single pageSizeMap as source of truth (no switch
duplication). Creator layer iota values mirror domain 1:1 via direct
cast — validated by compile-time structural test.

Tests cover all 35+ sizes, A-series halving invariant, JIS vs ISO
B-series distinction, presentation aspect ratios, and iota order guard.

Add document.AddPageWithRect for domain-level custom page creation.
Add creator.NewPageWithDimensions(widthPt, heightPt) for ergonomic
custom dimensions at the creator layer.

Both methods initialize all page fields identically to the existing
AddPage/NewPageWithSize methods. Negative and zero dimensions are
rejected with descriptive errors.

Unit conversion helpers (InchesToPoints, MMToPoints, CMToPoints) and
their constants are now also exported from the creator package,
delegating to the domain layer implementations to avoid duplication.

Tests cover: correct dimensions, validation errors, page count
increment, mixed use with standard sizes, PDF write round-trip,
default margins, and content area calculation.

Add Rotation float64 field to creator.TextOperation and writer.TextOp
structs. Rotation is in degrees, counter-clockwise, around the text
origin point (X, Y).

New creator.Page methods:
  AddTextRotated(text, x, y, font, size, rotation) error
  AddTextColorRotated(text, x, y, font, size, color, rotation) error

Content stream generation now uses the Tm (text matrix) PDF operator
when Rotation != 0, computing cos/sin from the degree value. Zero
rotation continues to use the Td (MoveTextPosition) operator, preserving
backward compatibility.

convertTextOps propagates the Rotation field from creator to writer.

Tests cover: 0/90/45/negative degree values, color and validation, Tm
vs Td operator selection, matrix math correctness, end-to-end PDF
write, multiple rotations on one page, and AddText regression guard.

…012)

…eWithSize

NewPageWithSize now accepts an optional orientation parameter — the
industry-standard approach used by PDFKit, jsPDF, gofpdf, TCPDF, and others.
Uses swapped-MediaBox (93% industry consensus), no /Rotate.

- Fix stale godoc: ToRectangle() referenced SetRotation for landscape,
  now points to NewPageWithSize with Landscape option
- Fix misleading comment: 'compile-time check' → 'runtime check'
- Add reverse conversion functions to creator layer (PointsToInches,
  PointsToMM, PointsToCM) for API symmetry with forward conversions
- Add domain-layer test for AddPageWithRect (was only tested indirectly)
- Fix rotation tests: now assert actual content stream matrix bytes
  instead of testing Go math library

- CHANGELOG: add v0.4.0 section (35+ page sizes, custom dimensions,
  landscape orientation, text rotation)
- README: add new features to Key Features section, update version plan
- ROADMAP: add v0.4.0 'Creator API' section, shift encryption/signatures
  to v0.5.0, update feature status table and backlog

- Add normalizeAngle() helper for consistent angle representation
- Apply normalization in AddTextColorRotated and AddTextCustomFontColorRotated
- Negative angles accepted: -90 becomes 270, -45 becomes 315
- Fractional angles (22.5°, 33.3°) fully supported
- Document CCW-positive convention per ISO 32000 §8.3
- Clarify Slide16x9/Slide4x3 are already landscape-oriented
- Note Orientation behavior with presentation sizes
- Add 16 normalization test cases + equivalence tests

feat: v0.4.0 Creator API — page sizes, custom dimensions, landscape, text rotation