Fix documentation inconsistencies: SPDX headers and docstrings

[Copilot]

What does this changes

Standardizes SPDX copyright headers and improves docstring clarity for Thai G2P transliteration modules.

What was wrong

• SPDX headers: Inconsistent copyright year formats (some files showed "2026" instead of "2016-2026"), trailing periods, and malformed entries with "Copyright" prefix
• Copyright blocks: The perceptron tagger had unformatted copyright lines outside the docstring
• Docstrings: Three G2P transliteration classes (ThaiG2P, Umt5ThaiG2P) had identical generic docstrings despite using different model architectures (PyTorch, transformers, UMT5)

How this fixes it

SPDX header fixes (5 files):

• pythainlp/tokenize/thai2fit.py, pythainlp/util/profanity.py: Changed 2026 → 2016-2026
• pythainlp/corpus/tnc.py: Removed trailing period from Project. → Project
• pythainlp/tokenize/thaisumcut.py: Changed Copyright 2020 → 2020 in SPDX field
• All files now follow standard order: FileCopyrightText, FileType, License-Identifier

Copyright block fix (1 file):

• pythainlp/tag/_tag_perceptron.py: Moved MIT license copyright statements into docstring

Docstring improvements (3 files):

• pythainlp/transliterate/thaig2p.py: Now specifies "PyTorch-based model (v1)"
• pythainlp/transliterate/thaig2p_v2.py: Now specifies "transformer-based model (v2)"
• pythainlp/transliterate/umt5_thaig2p.py: Now specifies "UMT5 model"

Each docstring now clearly describes the underlying architecture and links to relevant documentation.

Your checklist for this pull request

• Passed code styles and structures
• Passed code linting checks and unit test

Original prompt

Fix documentation, including code comments and docstring, inconsistencies across the repo, including formatting consistency.

Fact checks claims about external libraries if still valid.

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud