Convert all logging to structured message templates

[lahma]

Closes #265.

All ~890 ILogger call sites across src/, test/ and examples/ now use constant message templates with PascalCase named placeholders instead of string interpolation, concatenation or string.Format. Structured sinks (NLog ${message:raw=true}, Serilog, etc.) get real templates and named properties, and message-template-based categorization works for Spring logs.

Highlights

• Latent crash fix: ~97 sites passed dynamically built strings as the template. Text containing braces (SpEL literals {1,2,3}, ODBC escapes {call sp}, Velocity ${var}) was parsed as a malformed template. Pass-through adapters (SimpleLoggingAdvice, NVelocity CommonsLoggingLogSystem, NMS NmsTrace, FailFastProblemReporter, …) now log via a constant "{Message}" template; sites whose message was assembled next to the call got real templates.
• Credential leak fix: AdoPlatformTransactionManager/ConnectionUtils no longer log ConnectionString (the concern raised in Log messages use concatenated strings #265).
• Exception handling: sites that flattened exceptions into message text now use the exception-first overloads; two examples that passed the exception as an unused template argument are fixed; redundant (Exception) casts removed.
• Regression guard: CA2254/CA2253/CA2017/CA1727 are now build errors (EnableNETAnalyzers + AnalysisMode=None + editorconfig opt-in), so non-constant templates can't come back.
• MEL.Abstractions floor 2.1.0 → 8.0.0: 2.1.x built a formatter for any message containing { even with zero args (render-time FormatException) and had no template caching. NUnitAspEx-hosted test AppDomains needed System.Memory/Unsafe binding redirects in their web.configs as a consequence.
• 348 now-redundant IsEnabled guards removed (templates defer formatting); guards kept where argument evaluation is expensive, levels mismatch (verbosity gates), or contracts pin behavior (SimpleLoggingAdvice).
• Spring.Messaging.Ems also got its four orphaned ILog references fixed (the project did not compile since the ILog alias removal).

Rendered-output changes (documented in BreakingChanges.txt)

• Invariant culture instead of current culture for argument formatting
• null args render as (null)
• Exceptions render via the sink's exception slot instead of inline text
• A few collection args render as comma-separated lists
• Connection strings redacted as above

Verification

• Full solution rebuild with the four CA rules as errors: 0 warnings, 0 errors
• All locally runnable suites green on net8.0 + net462 (Spring.Web.Tests 202/202 incl. hosted-AppDomain fixtures); the one local failure is the machine-specific FormatUsingDefaults date-format quirk
• The out-of-order string.Format sites ({2}…{0}…{1}) were argument-reordered and hand-verified
• Spring.Messaging.Ems is excluded from the solution build (needs TIBCO.EMS.dll), so its conversions are verified by review plus a placeholder/argument arity lint (59/59 clean) — CI cannot compile this project
• Rendered text was kept byte-identical except where listed above; the two test fixtures that assert rendered log output pass unchanged

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