feat: add CJK predefined CMap decoders (Shift-JIS and UCS-2 BE)

[Detective-XH]

Problem

getEncoder() has no handler for several widely-used CJK predefined CMaps.
PDFs that declare one of the following encodings fall through to the default
branch and return garbled U+FFFD replacement characters instead of readable text:

• 90ms-RKSJ-H, 90ms-RKSJ-V, 90pv-RKSJ-H — Japanese Shift-JIS CMaps
  (Adobe-Japan1, used by virtually every Japanese PDF tool)
• UniGB-UCS2-H/V, UniCNS-UCS2-H/V, UniJIS-UCS2-H/V, UniKS-UCS2-H/V
  — the four Adobe Uni*-UCS2 CMap families (Simplified Chinese, Traditional
  Chinese, Japanese, Korean)

Fix

Two small, focused encoder types added to page.go:

multibyteCMapEncoder (Shift-JIS)
Wraps golang.org/x/text/encoding/japanese.ShiftJIS to decode the raw
content-stream bytes. x/text is already an indirect dependency of this
module, so no new transitive dependency is introduced.

ucs2BEEncoder (Uni*-UCS2-H/V)
Reads successive 2-byte big-endian values directly as Unicode BMP code points.
Requires no additional import — the Uni*-UCS2-* CMaps map each glyph selector
to a single BMP code point, making plain uint16 arithmetic correct and
sufficient.

Tests

page_cjk_test.go (new file) covers both encoders:

• TestUCS2BEEncoder — 7 sub-tests: Simplified Chinese, Traditional Chinese,
  Japanese hiragana, Korean hangul, ASCII round-trip, empty input, trailing
  odd byte.
• TestMultibyteCMapEncoder_ShiftJIS — 3 sub-tests: katakana, mixed
  kanji/hiragana, empty input.

All 10 tests pass (go test -race ./...).

Relation to existing PRs

PR #56 adds UniGB-UCS2-H with a ucs2Encoder using encoding/binary +
unicode/utf16. This PR extends that idea to all eight Uni*-UCS2 variants
and uses a simpler implementation (no standard-library imports beyond those
already present) while also adding the unrelated Shift-JIS family. If #56 is
preferred, this PR can be scoped to the Shift-JIS encoder only.

[Detective-XH]

Closing in favour of a more complete PR that covers all five CJK predefined CMap families (Shift-JIS, UCS-2 BE, GBK, Big5-ETen, UHC/KSCms) in one set.