feat: add dict-style hierarchy access and ambient clock domains

[cxzzzz]

Summary

Two related additions to how signals are located and loaded.

Dict-style hierarchy access

reader[path] returns the Signal or Scope at an exact path; nested
lookups (scope["a.b"]) resolve relative paths, and sig[7:4] bit-selects
like Waveform indexing.

signal = reader['tb.dut.data']
scope = reader['tb.dut']
low_byte = reader['tb.dut.data'][7:0]

Ambient clock domains

ClockDomain bundles a clock signal with a sampling recipe (edge, time or
cycle window), so it doesn't need to be repeated on every call:

with r.clock_domain(clock='tb.clk'):
    w_ptr = r['tb.dut.w_ptr'].w              # Signal.w — ambient shorthand
    r_ptr = r.load_waveform('tb.dut.r_ptr')  # clock omitted — same ambient domain

• Signal.w / Signal.m / Signal.waveform() / Signal.unknown_mask() read the ambient domain.
• load_waveform, load_unknown_mask, load_matched_waveforms, and load_matched_unknown_masks default clock/clock_path to None and fall back to the same ambient domain when omitted.

Docs

Rewrote the reader, signal-query, and waveform-analysis guides (EN + ZH) to lead with dict-style access and ambient clock domains, and added ClockDomain to the API reference.

Examples

Rewrote the six existing examples to use dict-style access and ambient clock domains throughout, and added a new multi_cluster_occupancy example demonstrating batch scope matching (get_matched_scopes with brace, recursive wildcard, and regex captures combined) across a bank of clusters.

Testing

Tests, lint, type-check, and doc build all pass, including the FSDB suite.