nil-compare: detect typed nil conversions

[mmorel-35]

The nil-compare checker failed to recognise typed nil conversions such as (*tls.Config)(nil), []MailserverTopic(nil), []base.DiskFileNum(nil) as nil values, so patterns like the following went undetected:

assert.EqualValues(t, (*tls.Config)(nil), opts.TLSConfig())  // should → assert.Nil(t, opts.TLSConfig())
require.EqualValues(t, []base.DiskFileNum(nil), recycler.LogNumsForTesting())

nil-compare

• Added isTypedNil(pass, expr) using pass.TypesInfo.Types[ce.Fun].IsType() to detect any T(nil) type-conversion expression.
• Updated xorNil (now accepts *analysis.Pass) to match both untyped nil and typed nil via the new isNilExpr helper.
• Typed nil on either side of the comparison is handled; the surviving non-nil argument is kept in the suggested fix.

expected-actual

• isExpectedValueCandidate previously treated (T)(nil) as an expected-value candidate via isParenExpr, causing false-positive "reverse arguments" diagnostics for comparison functions.
• For Equal, EqualValues, Exactly, NotEqual, NotEqualValues etc., typed nil is now excluded from the expected-value heuristic — nil-compare owns those cases.
• IsType / IsNotType are unaffected; (*T)(nil) as a type witness continues to be recognised correctly.

Tests

• gen_nil_compare.go: replaced the inert UnsupportedAssertions block with TypedNilAssertions carrying full report/proposed-fix metadata; both left- and right-side typed nil variants are covered for all five affected functions.
• gen_expected_actual.go: added ignored-assertion entries confirming typed nil in comparison functions does not trigger expected-actual.

Closes #222