Make views of TrackedArray return a TrackedArray

[devmotion]

Fixes #281.

Base.view had no TrackedArray method, so it returned a SubArray of TrackedReals — an array of individually tracked scalars rather than a single tracked array. Every subsequent operation then recorded one instruction per element, so the tape grew linearly with the size of the view:

d	x[2:end]	@view x[2:end]
10	2	8
100	2	98
1000	2	998

view now returns a TrackedArray backed by views of the parent's value and deriv arrays. Since the deriv array is shared with the parent, derivatives accumulate into the parent directly and no instruction has to be recorded at all — the tape length becomes 1 and is constant in d. Views that are not IndexLinear cannot back a TrackedArray and are materialized through getindex instead.

Gradient timings from the issue:

d	before	after
10	250 ns	41 ns
100	2666 ns	83 ns
1000	27750 ns	750 ns

This also fixes logical indexing in the generic getindex, which is a separate pre-existing bug: since Bool <: Integer, masks were routed there but never normalized, so false indexed element 0 and x[mask] threw a BoundsError. Indices are now normalized with Base.to_indices, which handles Colons and masks axis-aware and covers whole-array logical indexing such as m[mask]. The view fix relies on this, since logical views are not IndexLinear and so go through getindex.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.91%. Comparing base (bc2227e) to head (679f970).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #291      +/-   ##
==========================================
+ Coverage   85.10%   85.91%   +0.80%     
==========================================
  Files          18       18              
  Lines        1914     1945      +31     
==========================================
+ Hits         1629     1671      +42     
+ Misses        285      274      -11     

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