Split the licence so GitHub can read it

[Naruto]

This repository is public, and GitHub reports its licence as Other:

$ gh repo view cri-middleware/SSPlayerForGodot --json licenseInfo
{"licenseInfo":{"key":"other","name":"Other","nickname":""}}

LICENSE.md holds the BSD-3-Clause text followed by a Japanese translation of it
under a horizontal rule. GitHub's detector compares the whole file against known
licence texts; the translation is about 40% of it, so nothing matches within the
confidence threshold.

Consequences beyond the sidebar label: license:bsd-3-clause search never finds
the repository, dependency scanners and SBOM tooling that read the licence API
cannot classify it, and anyone checking licence compatibility has to open the
file and read it themselves.

Measured, not guessed

licensee 10.1.0 — the detector GitHub runs — against each candidate file.
Its match threshold is 98%.

LICENSE.md contains	Verdict	Similarity
the English text alone (this PR)	BSD-3-Clause	98.35%
the English text + an SPDX-License-Identifier line	NOASSERTION	92.25% (closest is BSD-4-Clause at 93.33%)
English + the Japanese translation (today)	NOASSERTION	57.53% at best
the English text alone, with LICENSE.ja.md beside it	BSD-3-Clause	98.35%

Two things this settles:

• The SPDX line has to stay out of the file. It is the standard machine-readable
  declaration and other tooling (REUSE, ScanCode, SBOM generators) does key off it,
  but licensee measures how close the file is to a known licence text — so a
  declaration line only dilutes it. One line costs six points and flips the verdict.
  The identifier belongs somewhere else; the SDK already carries it as
  license = "BSD-3-Clause" in Cargo.toml.
• LICENSE.ja.md sitting next to it changes nothing. The translation is safe
  where it is.

What changed

• LICENSE.md is now the BSD-3-Clause text alone.
• LICENSE.ja.md carries the translation, labelled a reference translation with
  the English named as authoritative.
• The leading SPDX-License-Identifier line is gone — see above. Nothing in the
  tree parsed it (git grep SPDX found only that line).

Two things fall out of the split:

• The locale licence pages stop being the same page. docs/ja/license.md and
  docs/en/license.md both --8<-- included LICENSE.md, so a Japanese reader
  got the English text with a translation appended and an English reader got the
  reverse. Each now includes its own file.
• Release packaging carries both. LICENSE.ja.md joins LICENSE.md in the
  addon root and in addons/spritestudio/licenses/, in all four build scripts,
  and in the archive check that proves they arrived.

Known: the margin is 0.35 points

98.35% against a threshold of 98%. The gap is clause 3, which names the company
where the canonical text says "the copyright holder"; restoring the canonical
wording measures 100.00%. Left alone here — that is a change to licence text, and
the same wording is in all ten repositories. Worth knowing that a later edit to
this file could drop it back under.

What this does not prove yet

GitHub runs licence detection against the default branch, so the label only
moves once this is merged. That is the point of doing it here first: this is the
one public repository, and the same LICENSE.md sits in all ten. If it turns into
BSD-3-Clause after merge, the same split rolls out to the other nine.

Checks

• pwsh parses both .ps1 scripts; bash -n / zsh -n parse both shell scripts.
• scripts/build-runtime.* still copies the SDK's own LICENSE.md into
  ss_player/runtime/. That file is the SDK's and is still bilingual — it follows
  when SpriteStudio-SDK gets the same treatment.