Add tolerant transcript alignment and ambiguous splicing-status layer

[PPSherry]

Summary

• Tolerant transcriptome alignment: add TxAlignOpts with micro_intron_length (accepts micro-introns within exons, default 50 bp) and junction_tolerance (accepts junction shifts, default off), plus terminal-exon end exemption. TxAlignOpts::exact() reproduces historical classifications.
• New ambiguous layer: reads whose splicing status conflicts across transcript annotations (e.g. exonic on one transcript, intronic on another) were previously dropped; they are now counted in a third ambiguous layer. Exposed as an AnnData layer and in QC metrics.
• Minor fixes and seqspec-guide doc update.

Testing

• Unit tests for terminal-exon exemption, micro-intron tolerance, junction tolerance, and the splicing-status partition invariant.