feat: add transaction input/output mutation methods

[shivv23]

Description

Adds 6 mutation methods to the Transaction class, directly addressing the TODO list priority item:

• add_input(txin) — append an input; auto-appends empty TxWitnessInput([]) when has_segwit=True
• add_output(txout) — append an output
• remove_input(index) — remove by index; also removes the corresponding witness entry for segwit transactions
• remove_output(index) — remove by index
• update_input(index, txin) — replace input at index
• update_output(index, txout) — replace output at index

All remove/update methods raise IndexError on out-of-bounds access, matching Python stdlib list semantics.

Testing

• 40 new tests in ests/test_transaction_mutation.py covering all 6 methods, round-trip serialization, segwit/non-segwit handling, empty transaction edge cases, negative indices, copy isolation, and multi-input witness management
• All 240 tests pass (200 existing + 40 new) with no regressions

Related

Closes the main TODO item: influences set_witness (add_input should add the empty witness immediately?)